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Frequently Asked Questions (FAQ) for PostgreSQL
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Fri Jan 4 00:44:42 EST 2002
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Last updated: Thu Jan 10 18:07:03 EST 2002
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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General Questions
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General Questions
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1.1) What is PostgreSQL?
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1.1) What is PostgreSQL? How is it pronounced?
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1.2) What's the copyright on PostgreSQL?
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1.2) What is the copyright on PostgreSQL?
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1.3) What Unix platforms does PostgreSQL run on?
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1.3) What Unix platforms does PostgreSQL run on?
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1.4) What non-unix ports are available?
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1.4) What non-Unix ports are available?
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1.5) Where can I get PostgreSQL?
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1.5) Where can I get PostgreSQL?
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1.6) Where can I get support?
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1.6) Where can I get support?
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1.7) What is the latest release?
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1.7) What is the latest release?
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1.11) Is PostgreSQL Y2K compliant?
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1.11) Is PostgreSQL Y2K compliant?
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1.12) How do I join the development team?
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1.12) How do I join the development team?
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1.13) How do I submit a bug report?
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1.13) How do I submit a bug report?
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1.14) How does PostgreSQL compare to other DBMS's?
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1.14) How does PostgreSQL compare to other DBMSs?
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1.15) How can I financially assist PostgreSQL?
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1.15) How can I financially assist PostgreSQL?
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User Client Questions
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User Client Questions
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2.1) Are there ODBC drivers for PostgreSQL?
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2.1) Are there ODBC drivers for PostgreSQL?
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2.2) What tools are available for use PostgreSQL with Web pages?
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2.2) What tools are available for using PostgreSQL with Web pages?
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2.3) Does PostgreSQL have a graphical user interface? A report
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2.3) Does PostgreSQL have a graphical user interface? A report
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generator? An embedded query language interface?
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generator? An embedded query language interface?
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2.4) What languages are available to communicate with PostgreSQL?
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2.4) What languages are available to communicate with PostgreSQL?
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3.1) How do I install PostgreSQL somewhere other than
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3.1) How do I install PostgreSQL somewhere other than
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/usr/local/pgsql?
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/usr/local/pgsql?
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3.2) When I start the postmaster, I get a Bad System Call or core
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3.2) When I start postmaster, I get a Bad System Call or core dumped
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dumped message. Why?
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message. Why?
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3.3) When I try to start the postmaster, I get IpcMemoryCreate errors.
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3.3) When I try to start postmaster, I get IpcMemoryCreate errors.
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Why?
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3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors.
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Why?
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Why?
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3.4) When I try to start the postmaster, I get IpcSemaphoreCreate
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errors. Why?
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3.5) How do I prevent other hosts from accessing my PostgreSQL
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3.5) How do I prevent other hosts from accessing my PostgreSQL
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database?
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database?
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3.6) Why can't I connect to my database from another machine?
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3.6) Why can't I connect to my database from another machine?
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3.7) How do I tune the database engine for better performance?
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3.7) How do I tune the database engine for better performance?
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3.8) What debugging features are available?
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3.8) What debugging features are available?
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3.9) I get "Sorry, too many clients" when trying to connect. Why?
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3.9) Why do I get "Sorry, too many clients" when trying to connect?
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3.10) What are the pg_sorttempNNN.NN files in my database directory?
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3.10) What are the pg_sorttempNNN.NN files in my database directory?
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Operational Questions
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Operational Questions
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4.2) How do I SELECT only the first few rows of a query?
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4.2) How do I SELECT only the first few rows of a query?
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4.3) How do I get a list of tables or other things I can see in psql?
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4.3) How do I get a list of tables or other things I can see in psql?
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4.4) How do you remove a column from a table?
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4.4) How do you remove a column from a table?
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4.5) What is the maximum size for a row, table, database?
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4.5) What is the maximum size for a row, a table, and a database?
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4.6) How much database disk space is required to store data from a
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4.6) How much database disk space is required to store data from a
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typical text file?
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typical text file?
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4.7) How do I find out what tables or indexes are defined in the
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4.7) How do I find out what tables or indexes are defined in the
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4.16) What is an OID? What is a TID?
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4.16) What is an OID? What is a TID?
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4.17) What is the meaning of some of the terms used in PostgreSQL?
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4.17) What is the meaning of some of the terms used in PostgreSQL?
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4.18) Why do I get the error "ERROR: Memory exhausted in
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4.18) Why do I get the error "ERROR: Memory exhausted in
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AllocSetAlloc()?"
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AllocSetAlloc()"?
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4.19) How do I tell what PostgreSQL version I am running?
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4.19) How do I tell what PostgreSQL version I am running?
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4.20) My large-object operations get invalid large obj descriptor.
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4.20) Why does my large-object operations get "invalid large obj
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Why?
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descriptor"?
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4.21) How do I create a column that will default to the current time?
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4.21) How do I create a column that will default to the current time?
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4.22) Why are my subqueries using IN so slow?
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4.22) Why are my subqueries using IN so slow?
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4.23) How do I perform an outer join?
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4.23) How do I perform an outer join?
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5.2) How can I contribute some nifty new types and functions to
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5.2) How can I contribute some nifty new types and functions to
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PostgreSQL?
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PostgreSQL?
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5.3) How do I write a C function to return a tuple?
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5.3) How do I write a C function to return a tuple?
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5.3) I have changed a source file. Why does the recompile not see the
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5.4) I have changed a source file. Why does the recompile not see the
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change?
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change?
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_________________________________________________________________
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_________________________________________________________________
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1.1) What is PostgreSQL?
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1.1) What is PostgreSQL?
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PostgreSQL is pronounced Post-Gres-Q-L.
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PostgreSQL is an enhancement of the POSTGRES database management
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PostgreSQL is an enhancement of the POSTGRES database management
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system, a next-generation DBMS research prototype. While PostgreSQL
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system, a next-generation DBMS research prototype. While PostgreSQL
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retains the powerful data model and rich data types of POSTGRES, it
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retains the powerful data model and rich data types of POSTGRES, it
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functionality was added in 1995, its name was changed to Postgres95.
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functionality was added in 1995, its name was changed to Postgres95.
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The name was changed at the end of 1996 to PostgreSQL.
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The name was changed at the end of 1996 to PostgreSQL.
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It is pronounced Post-Gres-Q-L.
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1.2) What is the copyright on PostgreSQL?
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1.2) What's the copyright on PostgreSQL?
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PostgreSQL is subject to the following COPYRIGHT:
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PostgreSQL is subject to the following COPYRIGHT:
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PostgreSQL Data Base Management System
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PostgreSQL Data Base Management System
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Portions copyright (c) 1996-2001, PostgreSQL Global Development Group
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Portions copyright (c) 1996-2002, PostgreSQL Global Development Group
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Portions Copyright (c) 1994-6 Regents of the University of California
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Portions Copyright (c) 1994-6 Regents of the University of California
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Permission to use, copy, modify, and distribute this software and its
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Permission to use, copy, modify, and distribute this software and its
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PostgreSQL. The platforms that had received explicit testing at the
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PostgreSQL. The platforms that had received explicit testing at the
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time of release are listed in the installation instructions.
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time of release are listed in the installation instructions.
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1.4) What non-unix ports are available?
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1.4) What non-Unix ports are available?
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Client
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Client
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1.5) Where can I get PostgreSQL?
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1.5) Where can I get PostgreSQL?
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The primary anonymous ftp site for PostgreSQL is
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The primary anonymous ftp site for PostgreSQL is
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ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main Web site.
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ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main web site.
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1.6) Where can I get support?
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1.6) Where can I get support?
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The main mailing list is: pgsql-general@PostgreSQL.org. It is
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The main mailing list is: pgsql-general@PostgreSQL.org. It is
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available for discussion of matters pertaining to PostgreSQL. To
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available for discussion of matters pertaining to PostgreSQL. To
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subscribe, send mail with the following lines in the body (not the
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subscribe, send mail with the following lines in the body (not the
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subject line)
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subject line):
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subscribe
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subscribe
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end
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end
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http://www.PostgreSQL.org
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http://www.PostgreSQL.org
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There is also an IRC channel on EFNet, channel #PostgreSQL. I use the
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There is also an IRC channel on EFNet, channel #PostgreSQL. I use the
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unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net.
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Unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net.
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A list of commercial support companies is available at
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A list of commercial support companies is available at
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http://www.postgresql.org/users-lounge/commercial-support.html.
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http://www.postgresql.org/users-lounge/commercial-support.html.
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psql has some nice \d commands to show information about types,
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psql has some nice \d commands to show information about types,
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operators, functions, aggregates, etc.
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operators, functions, aggregates, etc.
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Our Web site contains even more documentation.
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Our web site contains even more documentation.
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1.9) How do I find out about known bugs or missing features?
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1.9) How do I find out about known bugs or missing features?
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The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html
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The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html
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teaches SQL. There is a nice tutorial at
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teaches SQL. There is a nice tutorial at
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http://w3.one.net/~jhoffman/sqltut.htm and at
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http://www.intermedia.net/support/sql/sqltut.shtm and at
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http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM.
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http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM.
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Another one is "Teach Yourself SQL in 21 Days, Second Edition" at
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Another one is "Teach Yourself SQL in 21 Days, Second Edition" at
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1.11) Is PostgreSQL Y2K compliant?
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1.11) Is PostgreSQL Y2K compliant?
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Yes, we easily handle dates past the year 2000AD, and before 2000BC.
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Yes, we easily handle dates past the year 2000 AD, and before 2000 BC.
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1.12) How do I join the development team?
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1.12) How do I join the development team?
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First, download the latest source and read the PostgreSQL Developers
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First, download the latest source and read the PostgreSQL Developers
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documentation on our Web site, or in the distribution. Second,
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documentation on our web site, or in the distribution. Second,
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subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third,
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subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third,
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submit high-quality patches to pgsql-patches.
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submit high quality patches to pgsql-patches.
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There are about a dozen people who have commit privileges to the
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There are about a dozen people who have commit privileges to the
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PostgreSQL CVS archive. They each have submitted so many high-quality
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PostgreSQL CVS archive. They each have submitted so many high-quality
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1.13) How do I submit a bug report?
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1.13) How do I submit a bug report?
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Fill out the "bug-template" file and send it to:
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Please visit the PostgreSQL BugTool page, which gives guidelines and
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pgsql-bugs@PostgreSQL.org
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directions on how to submit a bug.
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Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
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Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
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there is a more recent PostgreSQL version or patches.
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there is a more recent PostgreSQL version or patches.
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1.14) How does PostgreSQL compare to other DBMS's?
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1.14) How does PostgreSQL compare to other DBMSs?
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There are several ways of measuring software: features, performance,
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There are several ways of measuring software: features, performance,
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reliability, support, and price.
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reliability, support, and price.
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Features
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Features
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PostgreSQL has most features present in large commercial
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PostgreSQL has most features present in large commercial DBMSs,
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DBMS's, like transactions, subselects, triggers, views, foreign
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like transactions, subselects, triggers, views, foreign key
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key referential integrity, and sophisticated locking. We have
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referential integrity, and sophisticated locking. We have some
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some features they don't have, like user-defined types,
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features they do not have, like user-defined types,
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inheritance, rules, and multi-version concurrency control to
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inheritance, rules, and multi-version concurrency control to
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reduce lock contention.
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reduce lock contention.
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data integrity within 30 seconds of an OS crash.
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data integrity within 30 seconds of an OS crash.
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In comparison to MySQL or leaner database systems, we are
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In comparison to MySQL or leaner database systems, we are
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slower on inserts/updates because we have transaction overhead.
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slower on inserts/updates because we have transaction overhead.
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Of course, MySQL doesn't have any of the features mentioned in
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Of course, MySQL does not have any of the features mentioned in
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the Features section above. We are built for flexibility and
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the Features section above. We are built for flexibility and
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features, though we continue to improve performance through
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features, though we continue to improve performance through
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profiling and source code analysis. There is an interesting Web
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profiling and source code analysis. There is an interesting Web
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http://openacs.org/why-not-mysql.html
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http://openacs.org/why-not-mysql.html
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We handle each user connection by creating a Unix process.
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We handle each user connection by creating a Unix process.
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Backend processes share data buffers and locking information.
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Backend processes share data buffers and locking information.
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With multiple CPU's, multiple backends can easily run on
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With multiple CPUs, multiple backends can easily run on
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different CPU's.
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different CPUs.
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Reliability
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Reliability
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We realize that a DBMS must be reliable, or it is worthless. We
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We realize that a DBMS must be reliable, or it is worthless. We
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Support
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Support
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Our mailing list provides a large group of developers and users
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Our mailing list provides a large group of developers and users
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to help resolve any problems encountered. While we can not
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to help resolve any problems encountered. While we can not
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guarantee a fix, commercial DBMS's don't always supply a fix
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guarantee a fix, commercial DBMSs do not always supply a fix
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either. Direct access to developers, the user community,
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either. Direct access to developers, the user community,
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manuals, and the source code often make PostgreSQL support
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manuals, and the source code often make PostgreSQL support
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superior to other DBMS's. There is commercial per-incident
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superior to other DBMSs. There is commercial per-incident
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support available for those who need it. (See support FAQ
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support available for those who need it. (See support FAQ
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item.)
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item.)
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1.15) How can I financially assist PostgreSQL?
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1.15) How can I financially assist PostgreSQL?
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PostgreSQL has had a first-class infrastructure since we started five
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PostgreSQL has had a first-class infrastructure since we started six
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years ago. This is all thanks to Marc Fournier, who has created and
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years ago. This is all thanks to Marc Fournier, who has created and
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managed this infrastructure over the years.
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managed this infrastructure over the years.
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They will probably be selling this product to people who need
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They will probably be selling this product to people who need
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commercial-quality support, but a freeware version will always be
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commercial-quality support, but a freeware version will always be
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available. Questions to postgres95@openlink.co.uk.
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available. Please send questions to postgres95@openlink.co.uk.
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See also the ODBC chapter of the Programmer's Guide.
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See also the ODBC chapter of the Programmer's Guide.
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2.2) What tools are available for using PostgreSQL with Web pages?
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2.2) What tools are available for using PostgreSQL with Web pages?
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A nice introduction to Database-backed Web pages can be seen at:
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A nice introduction to Database-backed Web pages can be seen at:
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http://www.webtools.com
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http://www.webreview.com
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There is also one at http://www.phone.net/home/mwm/hotlist/.
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There is also one at http://www.phone.net/home/mwm/hotlist/.
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For Web integration, PHP is an excellent interface. It is at
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For Web integration, PHP is an excellent interface. It is at
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http://www.php.net
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http://www.php.net.
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For complex cases, many use the Perl interface and CGI.pm.
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For complex cases, many use the Perl interface and CGI.pm.
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An embedded query language interface?
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An embedded query language interface?
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We have a nice graphical user interface called pgaccess, which is
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We have a nice graphical user interface called pgaccess, which is
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shipped as part of the distribution. Pgaccess also has a report
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shipped as part of the distribution. pgaccess also has a report
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generator. The Web page is http://www.flex.ro/pgaccess
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generator. The Web page is http://www.flex.ro/pgaccess
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We also include ecpg, which is an embedded SQL query language
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We also include ecpg, which is an embedded SQL query language
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Specify the --prefix option when running configure.
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Specify the --prefix option when running configure.
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3.2) When I start the postmaster, I get a Bad System Call or core dumped
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3.2) When I start postmaster, I get a Bad System Call or core dumped
|
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message. Why?
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message. Why?
|
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|
||||||
It could be a variety of problems, but first check to see that you
|
It could be a variety of problems, but first check to see that you
|
||||||
have System V extensions installed in your kernel. PostgreSQL requires
|
have System V extensions installed in your kernel. PostgreSQL requires
|
||||||
kernel support for shared memory and semaphores.
|
kernel support for shared memory and semaphores.
|
||||||
|
|
||||||
3.3) When I try to start the postmaster, I get IpcMemoryCreate errors. Why?
|
3.3) When I try to start postmaster, I get IpcMemoryCreate errors. Why?
|
||||||
|
|
||||||
You either do not have shared memory configured properly in your
|
You either do not have shared memory configured properly in your
|
||||||
kernel or you need to enlarge the shared memory available in the
|
kernel or you need to enlarge the shared memory available in the
|
||||||
kernel. The exact amount you need depends on your architecture and how
|
kernel. The exact amount you need depends on your architecture and how
|
||||||
many buffers and backend processes you configure for the postmaster.
|
many buffers and backend processes you configure for postmaster. For
|
||||||
For most systems, with default numbers of buffers and processes, you
|
most systems, with default numbers of buffers and processes, you need
|
||||||
need a minimum of ~1MB. See the PostgreSQL Administrator's Guide for
|
a minimum of ~1 MB. See the PostgreSQL Administrator's Guide for more
|
||||||
more detailed information about shared memory and semaphores.
|
detailed information about shared memory and semaphores.
|
||||||
|
|
||||||
3.4) When I try to start the postmaster, I get IpcSemaphoreCreate errors.
|
3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors. Why?
|
||||||
Why?
|
|
||||||
|
|
||||||
If the error message is IpcSemaphoreCreate: semget failed (No space
|
If the error message is IpcSemaphoreCreate: semget failed (No space
|
||||||
left on device) then your kernel is not configured with enough
|
left on device) then your kernel is not configured with enough
|
||||||
semaphores. Postgres needs one semaphore per potential backend
|
semaphores. Postgres needs one semaphore per potential backend
|
||||||
process. A temporary solution is to start the postmaster with a
|
process. A temporary solution is to start postmaster with a smaller
|
||||||
smaller limit on the number of backend processes. Use -N with a
|
limit on the number of backend processes. Use -N with a parameter less
|
||||||
parameter less than the default of 32. A more permanent solution is to
|
than the default of 32. A more permanent solution is to increase your
|
||||||
increase your kernel's SEMMNS and SEMMNI parameters.
|
kernel's SEMMNS and SEMMNI parameters.
|
||||||
|
|
||||||
|
Inoperative semaphores can also cause crashes during heavy database
|
||||||
|
access.
|
||||||
|
|
||||||
If the error message is something else, you might not have semaphore
|
If the error message is something else, you might not have semaphore
|
||||||
support configured in your kernel at all. See the PostgreSQL
|
support configured in your kernel at all. See the PostgreSQL
|
||||||
@ -478,47 +480,44 @@
|
|||||||
|
|
||||||
By default, PostgreSQL only allows connections from the local machine
|
By default, PostgreSQL only allows connections from the local machine
|
||||||
using Unix domain sockets. Other machines will not be able to connect
|
using Unix domain sockets. Other machines will not be able to connect
|
||||||
unless you add the -i flag to the postmaster, and enable host-based
|
unless you add the -i flag to postmaster, and enable host-based
|
||||||
authentication by modifying the file $PGDATA/pg_hba.conf accordingly.
|
authentication by modifying the file $PGDATA/pg_hba.conf accordingly.
|
||||||
This will allow TCP/IP connections.
|
This will allow TCP/IP connections.
|
||||||
|
|
||||||
Inoperative semaphores can also cause crashes during heavy database
|
|
||||||
access.
|
|
||||||
|
|
||||||
3.6) Why can't I connect to my database from another machine?
|
3.6) Why can't I connect to my database from another machine?
|
||||||
|
|
||||||
The default configuration allows only unix domain socket connections
|
The default configuration allows only Unix domain socket connections
|
||||||
from the local machine. To enable TCP/IP connections, make sure the
|
from the local machine. To enable TCP/IP connections, make sure
|
||||||
postmaster has been started with the -i option, and add an appropriate
|
postmaster has been started with the -i option, and add an appropriate
|
||||||
host entry to the file pgsql/data/pg_hba.conf.
|
host entry to the file pgsql/data/pg_hba.conf.
|
||||||
|
|
||||||
3.7) How do I tune the database engine for better performance?
|
3.7) How do I tune the database engine for better performance?
|
||||||
|
|
||||||
Certainly, indices can speed up queries. The EXPLAIN command allows
|
Certainly, indexes can speed up queries. The EXPLAIN command allows
|
||||||
you to see how PostgreSQL is interpreting your query, and which
|
you to see how PostgreSQL is interpreting your query, and which
|
||||||
indices are being used.
|
indexes are being used.
|
||||||
|
|
||||||
If you are doing a lot of INSERTs, consider doing them in a large
|
If you are doing many INSERTs, consider doing them in a large batch
|
||||||
batch using the COPY command. This is much faster than individual
|
using the COPY command. This is much faster than individual INSERTS.
|
||||||
INSERTS. Second, statements not in a BEGIN WORK/COMMIT transaction
|
Second, statements not in a BEGIN WORK/COMMIT transaction block are
|
||||||
block are considered to be in their own transaction. Consider
|
considered to be in their own transaction. Consider performing several
|
||||||
performing several statements in a single transaction block. This
|
statements in a single transaction block. This reduces the transaction
|
||||||
reduces the transaction overhead. Also consider dropping and
|
overhead. Also, consider dropping and recreating indexes when making
|
||||||
recreating indices when making large data changes.
|
large data changes.
|
||||||
|
|
||||||
There are several tuning options. You can disable fsync() by starting
|
There are several tuning options. You can disable fsync() by starting
|
||||||
the postmaster with a -o -F option. This will prevent fsync()'s from
|
postmaster with a -o -F option. This will prevent fsync()s from
|
||||||
flushing to disk after every transaction.
|
flushing to disk after every transaction.
|
||||||
|
|
||||||
You can also use the postmaster -B option to increase the number of
|
You can also use the postmaster -B option to increase the number of
|
||||||
shared memory buffers used by the backend processes. If you make this
|
shared memory buffers used by the backend processes. If you make this
|
||||||
parameter too high, the postmaster may not start because you've
|
parameter too high, the postmaster may not start because you have
|
||||||
exceeded your kernel's limit on shared memory space. Each buffer is 8K
|
exceeded your kernel's limit on shared memory space. Each buffer is 8K
|
||||||
and the default is 64 buffers.
|
and the default is 64 buffers.
|
||||||
|
|
||||||
You can also use the backend -S option to increase the maximum amount
|
You can also use the backend -S option to increase the maximum amount
|
||||||
of memory used by the backend process for temporary sorts. The -S
|
of memory used by the backend process for temporary sorts. The -S
|
||||||
value is measured in kilobytes, and the default is 512 (ie, 512K).
|
value is measured in kilobytes, and the default is 512 (i.e. 512K).
|
||||||
|
|
||||||
You can also use the CLUSTER command to group data in tables to match
|
You can also use the CLUSTER command to group data in tables to match
|
||||||
an index. See the CLUSTER manual page for more details.
|
an index. See the CLUSTER manual page for more details.
|
||||||
@ -529,12 +528,12 @@
|
|||||||
can be valuable for debugging purposes.
|
can be valuable for debugging purposes.
|
||||||
|
|
||||||
First, by running configure with the --enable-cassert option, many
|
First, by running configure with the --enable-cassert option, many
|
||||||
assert()'s monitor the progress of the backend and halt the program
|
assert()s monitor the progress of the backend and halt the program
|
||||||
when something unexpected occurs.
|
when something unexpected occurs.
|
||||||
|
|
||||||
Both postmaster and postgres have several debug options available.
|
Both postmaster and postgres have several debug options available.
|
||||||
First, whenever you start the postmaster, make sure you send the
|
First, whenever you start postmaster, make sure you send the standard
|
||||||
standard output and error to a log file, like:
|
output and error to a log file, like:
|
||||||
cd /usr/local/pgsql
|
cd /usr/local/pgsql
|
||||||
./bin/postmaster >server.log 2>&1 &
|
./bin/postmaster >server.log 2>&1 &
|
||||||
|
|
||||||
@ -545,18 +544,18 @@
|
|||||||
that specifies the debug level. Be warned that high debug level values
|
that specifies the debug level. Be warned that high debug level values
|
||||||
generate large log files.
|
generate large log files.
|
||||||
|
|
||||||
If the postmaster is not running, you can actually run the postgres
|
If postmaster is not running, you can actually run the postgres
|
||||||
backend from the command line, and type your SQL statement directly.
|
backend from the command line, and type your SQL statement directly.
|
||||||
This is recommended only for debugging purposes. Note that a newline
|
This is recommended only for debugging purposes. Note that a newline
|
||||||
terminates the query, not a semicolon. If you have compiled with
|
terminates the query, not a semicolon. If you have compiled with
|
||||||
debugging symbols, you can use a debugger to see what is happening.
|
debugging symbols, you can use a debugger to see what is happening.
|
||||||
Because the backend was not started from the postmaster, it is not
|
Because the backend was not started from postmaster, it is not running
|
||||||
running in an identical environment and locking/backend interaction
|
in an identical environment and locking/backend interaction problems
|
||||||
problems may not be duplicated.
|
may not be duplicated.
|
||||||
|
|
||||||
If the postmaster is running, start psql in one window, then find the
|
If postmaster is running, start psql in one window, then find the PID
|
||||||
PID of the postgres process used by psql. Use a debugger to attach to
|
of the postgres process used by psql. Use a debugger to attach to the
|
||||||
the postgres PID. You can set breakpoints in the debugger and issue
|
postgres PID. You can set breakpoints in the debugger and issue
|
||||||
queries from psql. If you are debugging postgres startup, you can set
|
queries from psql. If you are debugging postgres startup, you can set
|
||||||
PGOPTIONS="-W n", then start psql. This will cause startup to delay
|
PGOPTIONS="-W n", then start psql. This will cause startup to delay
|
||||||
for n seconds so you can attach to the process with the debugger, set
|
for n seconds so you can attach to the process with the debugger, set
|
||||||
@ -570,13 +569,13 @@
|
|||||||
pgsql/data/base/dbname directory. The client profile file will be put
|
pgsql/data/base/dbname directory. The client profile file will be put
|
||||||
in the client's current directory.
|
in the client's current directory.
|
||||||
|
|
||||||
3.9) I get 'Sorry, too many clients' when trying to connect. Why?
|
3.9) Why do I get "Sorry, too many clients" when trying to connect?
|
||||||
|
|
||||||
You need to increase the postmaster's limit on how many concurrent
|
You need to increase postmaster's limit on how many concurrent backend
|
||||||
backend processes it can start.
|
processes it can start.
|
||||||
|
|
||||||
The default limit is 32 processes. You can increase it by restarting
|
The default limit is 32 processes. You can increase it by restarting
|
||||||
the postmaster with a suitable -N value or modifying postgresql.conf.
|
postmaster with a suitable -N value or modifying postgresql.conf.
|
||||||
|
|
||||||
Note that if you make -N larger than 32, you must also increase -B
|
Note that if you make -N larger than 32, you must also increase -B
|
||||||
beyond its default of 64; -B must be at least twice -N, and probably
|
beyond its default of 64; -B must be at least twice -N, and probably
|
||||||
@ -639,13 +638,13 @@
|
|||||||
DROP TABLE old_table;
|
DROP TABLE old_table;
|
||||||
ALTER TABLE new_table RENAME TO old_table;
|
ALTER TABLE new_table RENAME TO old_table;
|
||||||
|
|
||||||
4.5) What is the maximum size for a row, table, database?
|
4.5) What is the maximum size for a row, a table, and a database?
|
||||||
|
|
||||||
These are the limits:
|
These are the limits:
|
||||||
Maximum size for a database? unlimited (60GB databases exist)
|
Maximum size for a database? unlimited (60 GB databases exist)
|
||||||
Maximum size for a table? 16 TB
|
Maximum size for a table? 16 TB
|
||||||
Maximum size for a row? unlimited in 7.1 and later
|
Maximum size for a row? unlimited in 7.1 and later
|
||||||
Maximum size for a field? 1GB in 7.1 and later
|
Maximum size for a field? 1 GB in 7.1 and later
|
||||||
Maximum number of rows in a table? unlimited
|
Maximum number of rows in a table? unlimited
|
||||||
Maximum number of columns in a table? 250-1600 depending on column types
|
Maximum number of columns in a table? 250-1600 depending on column types
|
||||||
Maximum number of indexes on a table? unlimited
|
Maximum number of indexes on a table? unlimited
|
||||||
@ -654,8 +653,8 @@
|
|||||||
disk space and memory/swap space. Performance may suffer when these
|
disk space and memory/swap space. Performance may suffer when these
|
||||||
values get unusually large.
|
values get unusually large.
|
||||||
|
|
||||||
The maximum table size of 16TB does not require large file support
|
The maximum table size of 16 TB does not require large file support
|
||||||
from the operating system. Large tables are stored as multiple 1GB
|
from the operating system. Large tables are stored as multiple 1 GB
|
||||||
files so file system size limits are not important.
|
files so file system size limits are not important.
|
||||||
|
|
||||||
The maximum table size and maximum number of columns can be increased
|
The maximum table size and maximum number of columns can be increased
|
||||||
@ -668,8 +667,8 @@
|
|||||||
required to store the data in a flat file.
|
required to store the data in a flat file.
|
||||||
|
|
||||||
Consider a file of 300,000 lines with two integers on each line. The
|
Consider a file of 300,000 lines with two integers on each line. The
|
||||||
flat file is 2.4MB. The size of the PostgreSQL database file
|
flat file is 2.4 MB. The size of the PostgreSQL database file
|
||||||
containing this data can be estimated at 14MB:
|
containing this data can be estimated at 14 MB:
|
||||||
36 bytes: each row header (approximate)
|
36 bytes: each row header (approximate)
|
||||||
+ 8 bytes: two int fields @ 4 bytes each
|
+ 8 bytes: two int fields @ 4 bytes each
|
||||||
+ 4 bytes: pointer on page to tuple
|
+ 4 bytes: pointer on page to tuple
|
||||||
@ -686,7 +685,7 @@
|
|||||||
-------------------- = 1755 database pages
|
-------------------- = 1755 database pages
|
||||||
171 rows per page
|
171 rows per page
|
||||||
|
|
||||||
1755 database pages * 8192 bytes per page = 14,376,960 bytes (14MB)
|
1755 database pages * 8192 bytes per page = 14,376,960 bytes (14 MB)
|
||||||
|
|
||||||
Indexes do not require as much overhead, but do contain the data that
|
Indexes do not require as much overhead, but do contain the data that
|
||||||
is being indexed, so they can be large also.
|
is being indexed, so they can be large also.
|
||||||
@ -705,7 +704,7 @@
|
|||||||
PostgreSQL does not automatically maintain statistics. VACUUM must be
|
PostgreSQL does not automatically maintain statistics. VACUUM must be
|
||||||
run to update the statistics. After statistics are updated, the
|
run to update the statistics. After statistics are updated, the
|
||||||
optimizer knows how many rows in the table, and can better decide if
|
optimizer knows how many rows in the table, and can better decide if
|
||||||
it should use indices. Note that the optimizer does not use indices in
|
it should use indexes. Note that the optimizer does not use indexes in
|
||||||
cases when the table is small because a sequential scan would be
|
cases when the table is small because a sequential scan would be
|
||||||
faster.
|
faster.
|
||||||
|
|
||||||
@ -720,9 +719,9 @@
|
|||||||
followed by an explicit sort is faster than an indexscan of all tuples
|
followed by an explicit sort is faster than an indexscan of all tuples
|
||||||
of a large table. This is because random disk access is very slow.
|
of a large table. This is because random disk access is very slow.
|
||||||
|
|
||||||
When using wild-card operators such as LIKE or ~, indices can only be
|
When using wild-card operators such as LIKE or ~, indexes can only be
|
||||||
used if the beginning of the search is anchored to the start of the
|
used if the beginning of the search is anchored to the start of the
|
||||||
string. So, to use indices, LIKE searches should not begin with %, and
|
string. So, to use indexes, LIKE searches should not begin with %, and
|
||||||
~(regular expression searches) should start with ^.
|
~(regular expression searches) should start with ^.
|
||||||
|
|
||||||
4.9) How do I see how the query optimizer is evaluating my query?
|
4.9) How do I see how the query optimizer is evaluating my query?
|
||||||
@ -733,7 +732,7 @@
|
|||||||
|
|
||||||
An R-tree index is used for indexing spatial data. A hash index can't
|
An R-tree index is used for indexing spatial data. A hash index can't
|
||||||
handle range searches. A B-tree index only handles range searches in a
|
handle range searches. A B-tree index only handles range searches in a
|
||||||
single dimension. R-tree's can handle multi-dimensional data. For
|
single dimension. R-trees can handle multi-dimensional data. For
|
||||||
example, if an R-tree index can be built on an attribute of type
|
example, if an R-tree index can be built on an attribute of type
|
||||||
point, the system can more efficiently answer queries such as "select
|
point, the system can more efficiently answer queries such as "select
|
||||||
all points within a bounding rectangle."
|
all points within a bounding rectangle."
|
||||||
@ -741,8 +740,8 @@
|
|||||||
The canonical paper that describes the original R-tree design is:
|
The canonical paper that describes the original R-tree design is:
|
||||||
|
|
||||||
Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
|
Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
|
||||||
Searching." Proc of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data,
|
Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt of
|
||||||
45-57.
|
Data, 45-57.
|
||||||
|
|
||||||
You can also find this paper in Stonebraker's "Readings in Database
|
You can also find this paper in Stonebraker's "Readings in Database
|
||||||
Systems".
|
Systems".
|
||||||
@ -771,15 +770,13 @@
|
|||||||
FROM tab
|
FROM tab
|
||||||
WHERE lower(col) = 'abc'
|
WHERE lower(col) = 'abc'
|
||||||
|
|
||||||
|
|
||||||
This will not use an standard index. However, if you create a
|
This will not use an standard index. However, if you create a
|
||||||
functional index, it will be used:
|
functional index, it will be used:
|
||||||
CREATE INDEX tabindex on tab (lower(col));
|
CREATE INDEX tabindex on tab (lower(col));
|
||||||
|
|
||||||
|
|
||||||
4.13) In a query, how do I detect if a field is NULL?
|
4.13) In a query, how do I detect if a field is NULL?
|
||||||
|
|
||||||
You test the column with IS NULLIS NOT NULL.
|
You test the column with IS NULL and IS NOT NULL.
|
||||||
|
|
||||||
4.14) What is the difference between the various character types?
|
4.14) What is the difference between the various character types?
|
||||||
|
|
||||||
@ -828,22 +825,20 @@ BYTEA bytea variable-length byte array (null-safe)
|
|||||||
However, if you need to dump and reload the database, you need to use
|
However, if you need to dump and reload the database, you need to use
|
||||||
pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
|
pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
|
||||||
|
|
||||||
Numbering Rows.
|
|
||||||
|
|
||||||
4.15.2) How do I get the value of a SERIAL insert?
|
4.15.2) How do I get the value of a SERIAL insert?
|
||||||
|
|
||||||
One approach is to to retrieve the next SERIAL value from the sequence
|
One approach is to retrieve the next SERIAL value from the sequence
|
||||||
object with the nextval() function before inserting and then insert it
|
object with the nextval() function before inserting and then insert it
|
||||||
explicitly. Using the example table in 4.15.1, that might look like
|
explicitly. Using the example table in 4.15.1, that might look like
|
||||||
this in Perl:
|
this in Perl:
|
||||||
new_id = output of "SELECT nextval('person_id_seq')"
|
new_id = output of "SELECT nextval('person_id_seq')"
|
||||||
INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
|
INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
|
||||||
|
|
||||||
You would then also have the new value stored in $newSerialID for use
|
You would then also have the new value stored in new_id for use in
|
||||||
in other queries (e.g., as a foreign key to the person table). Note
|
other queries (e.g., as a foreign key to the person table). Note that
|
||||||
that the name of the automatically created SEQUENCE object will be
|
the name of the automatically created SEQUENCE object will be named
|
||||||
named <table>_<serialcolumn>_seq, where table and serialcolumn are the
|
<table>_<serialcolumn>_seq, where table and serialcolumn are the names
|
||||||
names of your table and your SERIAL column, respectively.
|
of your table and your SERIAL column, respectively.
|
||||||
|
|
||||||
Alternatively, you could retrieve the assigned SERIAL value with the
|
Alternatively, you could retrieve the assigned SERIAL value with the
|
||||||
currval() function after it was inserted by default, e.g.,
|
currval() function after it was inserted by default, e.g.,
|
||||||
@ -877,10 +872,10 @@ BYTEA bytea variable-length byte array (null-safe)
|
|||||||
OID values. You can create an index on the OID field for faster
|
OID values. You can create an index on the OID field for faster
|
||||||
access.
|
access.
|
||||||
|
|
||||||
Oids are assigned to all new rows from a central area that is used by
|
OIDs are assigned to all new rows from a central area that is used by
|
||||||
all databases. If you want to change the OID to something else, or if
|
all databases. If you want to change the OID to something else, or if
|
||||||
you want to make a copy of the table, with the original OID's, there
|
you want to make a copy of the table, with the original OIDs, there is
|
||||||
is no reason you can't do it:
|
no reason you can't do it:
|
||||||
CREATE TABLE new_table(old_oid oid, mycol int);
|
CREATE TABLE new_table(old_oid oid, mycol int);
|
||||||
SELECT old_oid, mycol INTO new FROM old;
|
SELECT old_oid, mycol INTO new FROM old;
|
||||||
COPY new TO '/tmp/pgtable';
|
COPY new TO '/tmp/pgtable';
|
||||||
@ -892,7 +887,7 @@ BYTEA bytea variable-length byte array (null-safe)
|
|||||||
removed before anyone does.
|
removed before anyone does.
|
||||||
|
|
||||||
TIDs are used to identify specific physical rows with block and offset
|
TIDs are used to identify specific physical rows with block and offset
|
||||||
values. Tids change after rows are modified or reloaded. They are used
|
values. TIDs change after rows are modified or reloaded. They are used
|
||||||
by index entries to point to physical rows.
|
by index entries to point to physical rows.
|
||||||
|
|
||||||
4.17) What is the meaning of some of the terms used in PostgreSQL?
|
4.17) What is the meaning of some of the terms used in PostgreSQL?
|
||||||
@ -912,12 +907,12 @@ BYTEA bytea variable-length byte array (null-safe)
|
|||||||
A list of general database terms can be found at:
|
A list of general database terms can be found at:
|
||||||
http://www.comptechnews.com/~reaster/dbdesign.html
|
http://www.comptechnews.com/~reaster/dbdesign.html
|
||||||
|
|
||||||
4.18) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()?"
|
4.18) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
|
||||||
|
|
||||||
If you are running a version older than 7.1, an upgrade may fix the
|
If you are running a version older than 7.1, an upgrade may fix the
|
||||||
problem. Also it is possible you have run out of virtual memory on
|
problem. Also it is possible you have run out of virtual memory on
|
||||||
your system, or your kernel has a low limit for certain resources. Try
|
your system, or your kernel has a low limit for certain resources. Try
|
||||||
this before starting the postmaster:
|
this before starting postmaster:
|
||||||
ulimit -d 262144
|
ulimit -d 262144
|
||||||
limit datasize 256m
|
limit datasize 256m
|
||||||
|
|
||||||
@ -932,7 +927,8 @@ BYTEA bytea variable-length byte array (null-safe)
|
|||||||
|
|
||||||
From psql, type select version();
|
From psql, type select version();
|
||||||
|
|
||||||
4.20) My large-object operations get invalid large obj descriptor. Why?
|
4.20) Why does my large-object operations get "invalid large obj
|
||||||
|
descriptor"?
|
||||||
|
|
||||||
You need to put BEGIN WORK and COMMIT around any use of a large object
|
You need to put BEGIN WORK and COMMIT around any use of a large object
|
||||||
handle, that is, surrounding lo_open ... lo_close.
|
handle, that is, surrounding lo_open ... lo_close.
|
||||||
|
1115
doc/src/FAQ/FAQ.html
1115
doc/src/FAQ/FAQ.html
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