Update FAQ by eliminating non-frequent items like large objects and
extending questions. Update wording of various entries.
This commit is contained in:
Bruce Momjian
parent
48e7a196ea
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doc/FAQ
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doc/FAQ
@ -1,7 +1,7 @@
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Sat Apr 23 14:59:01 EDT 2005
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Last updated: Sat Apr 23 16:49:43 EDT 2005
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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@ -67,29 +67,17 @@
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4.11.3) Doesn't currval() lead to a race condition with other users?
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4.11.4) Why aren't my sequence numbers reused on transaction abort?
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Why are there gaps in the numbering of my sequence/SERIAL column?
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4.12) What is an OID? What is a TID?
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4.12) What is an OID? What is a CTID?
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4.13) Why do I get the error "ERROR: Memory exhausted in
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AllocSetAlloc()"?
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4.14) How do I tell what PostgreSQL version I am running?
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4.15) Why does my large-object operations get "invalid large obj
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descriptor"?
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4.16) How do I create a column that will default to the current time?
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4.17) How do I perform an outer join?
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4.18) How do I perform queries using multiple databases?
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4.19) How do I return multiple rows or columns from a function?
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4.20) Why do I get "relation with OID ##### does not exist" errors
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4.15) How do I create a column that will default to the current time?
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4.16) How do I perform an outer join?
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4.17) How do I perform queries using multiple databases?
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4.18) How do I return multiple rows or columns from a function?
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4.19) Why do I get "relation with OID ##### does not exist" errors
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when accessing temporary tables in PL/PgSQL functions?
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4.21) What encryption options are available?
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Extending PostgreSQL
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5.1) I wrote a user-defined function. When I run it in psql, why does
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it dump core?
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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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5.3) How do I write a C function to return a tuple?
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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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4.20) What encryption options are available?
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_________________________________________________________________
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General Questions
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@ -529,8 +517,8 @@
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LIMIT 1;
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If you believe the optimizer is incorrect in choosing a sequential
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scan, use SET enable_seqscan TO 'off' and run tests to see if an index
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scan is indeed faster.
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scan, use SET enable_seqscan TO 'off' and run query again to see if an
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index scan is indeed faster.
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When using wild-card operators such as LIKE or ~, indexes can only be
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used in certain circumstances:
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@ -576,13 +564,13 @@
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4.9) In a query, how do I detect if a field is NULL? How can I sort on
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whether a field is NULL or not?
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You test the column with IS NULL and IS NOT NULL.
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You test the column with IS NULL and IS NOT NULL, like this:
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SELECT *
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FROM tab
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WHERE col IS NULL;
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To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers
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in your WHERE clause. Things that are true will sort higher than
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in your ORDER BY clause. Things that are true will sort higher than
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things that are false, so the following will put NULL entries at the
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top of the resulting list:
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SELECT *
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@ -603,9 +591,9 @@
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The first four types above are "varlena" types (i.e., the first four
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bytes on disk are the length, followed by the data). Thus the actual
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space used is slightly greater than the declared size. However, these
|
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data types are also subject to compression or being stored out-of-line
|
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by TOAST, so the space on disk might also be less than expected.
|
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space used is slightly greater than the declared size. However, long
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values are also subject to compression, so the space on disk might
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also be less than expected.
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VARCHAR(n) is best when storing variable-length strings and it limits
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how long a string can be. TEXT is for strings of unlimited length,
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with a maximum of one gigabyte.
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@ -647,8 +635,8 @@
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You would then also have the new value stored in new_id for use in
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other queries (e.g., as a foreign key to the person table). Note that
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the name of the automatically created SEQUENCE object will be named
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<table>_<serialcolumn>_seq, where table and serialcolumn are the names
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of your table and your SERIAL column, respectively.
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<table>_< serialcolumn>_seq, where table and serialcolumn are the
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names of your table and your SERIAL column, respectively.
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Alternatively, you could retrieve the assigned SERIAL value with the
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currval() function after it was inserted by default, e.g.,
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@ -667,7 +655,7 @@
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transactions as needed and are not locked until the transaction
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completes. This causes gaps in numbering from aborted transactions.
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4.12) What is an OID? What is a TID?
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4.12) What is an OID? What is a CTID?
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Every row that is created in PostgreSQL gets a unique OID unless
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created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
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@ -680,9 +668,9 @@
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single table. and are therefore less likely to overflow. SERIAL8 is
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available for storing eight-byte sequence values.
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TIDs are used to identify specific physical rows with block and offset
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values. TIDs change after rows are modified or reloaded. They are used
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by index entries to point to physical rows.
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CTIDs are used to identify specific physical rows with block and
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offset values. CTIDs change after rows are modified or reloaded. They
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are used by index entries to point to physical rows.
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4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
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@ -703,26 +691,12 @@
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From psql, type SELECT version();
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4.15) Why does my large-object operations get "invalid large obj descriptor"?
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You need to put BEGIN WORK and COMMIT around any use of a large object
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handle, that is, surrounding lo_open ... lo_close.
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Currently PostgreSQL enforces the rule by closing large object handles
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at transaction commit. So the first attempt to do anything with the
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handle will draw invalid large obj descriptor. So code that used to
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work (at least most of the time) will now generate that error message
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if you fail to use a transaction.
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If you are using a client interface like ODBC you may need to set
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auto-commit off.
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4.16) How do I create a column that will default to the current time?
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4.15) How do I create a column that will default to the current time?
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Use CURRENT_TIMESTAMP:
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CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
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4.17) How do I perform an outer join?
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4.16) How do I perform an outer join?
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PostgreSQL supports outer joins using the SQL standard syntax. Here
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are two examples:
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@ -740,7 +714,7 @@
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is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called
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INNER joins.
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4.18) How do I perform queries using multiple databases?
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4.17) How do I perform queries using multiple databases?
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There is no way to query a database other than the current one.
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Because PostgreSQL loads database-specific system catalogs, it is
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@ -750,13 +724,13 @@
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course, a client can also make simultaneous connections to different
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databases and merge the results on the client side.
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4.19) How do I return multiple rows or columns from a function?
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4.18) How do I return multiple rows or columns from a function?
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It is easy using set-returning functions,
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http://techdocs.postgresql.org/guides/SetReturningFunctions
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.
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4.20) Why do I get "relation with OID ##### does not exist" errors when
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4.19) Why do I get "relation with OID ##### does not exist" errors when
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accessing temporary tables in PL/PgSQL functions?
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PL/PgSQL caches function scripts, and an unfortunate side effect is
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@ -767,7 +741,7 @@
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table access in PL/PgSQL. This will cause the query to be reparsed
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every time.
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4.21) What encryption options are available?
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4.20) What encryption options are available?
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* contrib/pgcrypto contains many encryption functions for use in SQL
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queries.
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@ -779,33 +753,4 @@
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or ssh, rather than PostgreSQL's native SSL connections.)
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* Database user passwords are automatically encrypted when stored in
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the system tables.
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* The server can run using an encrypted file system.
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_________________________________________________________________
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Extending PostgreSQL
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5.1) I wrote a user-defined function. When I run it in psql, why does it dump
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core?
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The problem could be a number of things. Try testing your user-defined
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function in a stand-alone test program first.
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5.2) How can I contribute some nifty new types and functions to PostgreSQL?
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Send your extensions to the pgsql-hackers mailing list, and they will
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eventually end up in the contrib/ subdirectory.
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5.3) How do I write a C function to return a tuple?
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In versions of PostgreSQL beginning with 7.3, table-returning
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functions are fully supported in C, PL/PgSQL, and SQL. See the
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Programmer's Guide for more information. An example of a
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table-returning function defined in C can be found in
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contrib/tablefunc.
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5.4) I have changed a source file. Why does the recompile not see the change?
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The Makefiles do not have the proper dependencies for include files.
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You have to do a make clean and then another make. If you are using
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GCC you can use the --enable-depend option of configure to have the
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compiler compute the dependencies automatically.
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* The server can also run using an encrypted file system.
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@ -10,7 +10,7 @@
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alink="#0000ff">
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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
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<P>Last updated: Sat Apr 23 14:59:01 EDT 2005</P>
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<P>Last updated: Sat Apr 23 16:49:43 EDT 2005</P>
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<P>Current maintainer: Bruce Momjian (<A href=
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"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
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@ -99,36 +99,24 @@
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reused on transaction abort? Why are there gaps in the numbering of
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my sequence/SERIAL column?<BR>
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<A href="#4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a
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<SMALL>TID</SMALL>?<BR>
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<A href="#4.12">4.13</A>) Why do I get the error <I>"ERROR: Memory
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<SMALL>CTID</SMALL>?<BR>
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<A href="#4.13">4.13</A>) Why do I get the error <I>"ERROR: Memory
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exhausted in AllocSetAlloc()"</I>?<BR>
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<A href="#4.14">4.14</A>) How do I tell what PostgreSQL version I
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am running?<BR>
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<A href="#4.15">4.15</A>) Why does my large-object operations get
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<I>"invalid large obj descriptor"</I>?<BR>
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<A href="#4.16">4.16</A>) How do I create a column that will
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<A href="#4.15">4.15</A>) How do I create a column that will
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default to the current time?<BR>
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<A href="#4.17">4.17</A>) How do I perform an outer join?<BR>
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<A href="#4.18">4.18</A>) How do I perform queries using multiple
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<A href="#4.16">4.16</A>) How do I perform an outer join?<BR>
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<A href="#4.17">4.17</A>) How do I perform queries using multiple
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databases?<BR>
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<A href="#4.19">4.19</A>) How do I return multiple rows or columns
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<A href="#4.18">4.18</A>) How do I return multiple rows or columns
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from a function?<BR>
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<A href="#4.20">4.20</A>) Why do I get "relation with OID #####
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<A href="#4.19">4.19</A>) Why do I get "relation with OID #####
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does not exist" errors when accessing temporary tables in PL/PgSQL
|
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functions?<BR>
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<A href="#4.21">4.21</A>) What encryption options are available?<BR>
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<A href="#4.20">4.20</A>) What encryption options are available?<BR>
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<H2 align="center">Extending PostgreSQL</H2>
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<A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
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it in <I>psql</I>, why does it dump core?<BR>
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<A href="#5.2">5.2</A>) How can I contribute some nifty new types
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and functions to PostgreSQL?<BR>
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<A href="#5.3">5.3</A>) How do I write a C function to return a
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tuple?<BR>
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<A href="#5.4">5.4</A>) I have changed a source file. Why does the
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recompile not see the change?<BR>
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<HR>
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<H2 align="center">General Questions</H2>
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@ -683,7 +671,7 @@ table?</TD><TD>unlimited</TD></TR>
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<P>If you believe the optimizer is incorrect in choosing a
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sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
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run tests to see if an index scan is indeed faster.</P>
|
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run query again to see if an index scan is indeed faster.</P>
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<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
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<I>~</I>, indexes can only be used in certain circumstances:</P>
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@ -733,7 +721,6 @@ table?</TD><TD>unlimited</TD></TR>
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FROM tab
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WHERE lower(col) = 'abc';
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</PRE>
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This will not use an standard index. However, if you create a
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expresssion index, it will be used:
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<PRE>
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@ -745,7 +732,7 @@ table?</TD><TD>unlimited</TD></TR>
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NULL</SMALL> or not?</H3>
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<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
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NOT NULL</SMALL>.</P>
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NOT NULL</SMALL>, like this:</P>
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<PRE>
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SELECT *
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@ -754,7 +741,7 @@ table?</TD><TD>unlimited</TD></TR>
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</PRE>
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<P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL>
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and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>WHERE</SMALL> clause.
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and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>ORDER BY</SMALL> clause.
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Things that are <I>true</I> will sort higher than things that are <I>false</I>,
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so the following will put NULL entries at the top of the resulting list:</P>
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@ -787,9 +774,8 @@ length</TD></TR>
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<P>The first four types above are "varlena" types (i.e., the first
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four bytes on disk are the length, followed by the data). Thus the
|
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actual space used is slightly greater than the declared size.
|
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However, these data types are also subject to compression or being
|
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stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
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might also be less than expected.</P>
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However, long values are also subject to compression, so the space
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on disk might also be less than expected.</P>
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<SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
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strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
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@ -805,8 +791,7 @@ length</TD></TR>
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serial/auto-incrementing field?</H3>
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<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
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auto-creates a sequence. For example,
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this:</P>
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auto-creates a sequence. For example, this:</P>
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<PRE>
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CREATE TABLE person (
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id SERIAL,
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@ -815,11 +800,12 @@ length</TD></TR>
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</PRE>
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is automatically translated into this:
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<PRE>
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CREATE SEQUENCE person_id_seq;
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CREATE TABLE person (
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id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
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name TEXT
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name TEXT
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);
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</PRE>
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@ -839,17 +825,17 @@ length</TD></TR>
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execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
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</PRE>
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You would then also have the new value stored in
|
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<CODE>new_id</CODE> for use in other queries (e.g., as a foreign
|
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key to the <CODE>person</CODE> table). Note that the name of the
|
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automatically created <SMALL>SEQUENCE</SMALL> object will be named
|
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<<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
|
||||
<I>table</I> and <I>serialcolumn</I> are the names of your table
|
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and your <SMALL>SERIAL</SMALL> column, respectively.
|
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You would then also have the new value stored in <CODE>new_id</CODE>
|
||||
for use in other queries (e.g., as a foreign key to the <CODE>person
|
||||
</CODE> table). Note that the name of the automatically created
|
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<SMALL>SEQUENCE</SMALL> object will be named <<I>table</I>>_<<I>
|
||||
serialcolumn</I>>_<I>seq</I>, where <I>table</I> and <I>serialcolumn</I>
|
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are the names of your table and your <SMALL>SERIAL</SMALL> column,
|
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respectively.
|
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|
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<P>Alternatively, you could retrieve the assigned
|
||||
<SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
|
||||
<I>after</I> it was inserted by default, e.g.,</P>
|
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<P>Alternatively, you could retrieve the assigned <SMALL>SERIAL</SMALL>
|
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value with the <I>currval()</I> function <I>after</I> it was inserted by
|
||||
default, e.g.,</P>
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<PRE>
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execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
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new_id = execute("SELECT currval('person_id_seq')");
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@ -871,7 +857,7 @@ length</TD></TR>
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transactions.</P>
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|
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<H3><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is
|
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a <SMALL>TID</SMALL>?</H3>
|
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a <SMALL>CTID</SMALL>?</H3>
|
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|
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<P>Every row that is created in PostgreSQL gets a unique
|
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<SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>.
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@ -888,8 +874,8 @@ length</TD></TR>
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<SMALL>SERIAL8</SMALL> is available for storing eight-byte sequence
|
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values.</P>
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<P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
|
||||
with block and offset values. T<SMALL>ID</SMALL>s change after rows
|
||||
<P>C<SMALL>TID</SMALL>s are used to identify specific physical rows
|
||||
with block and offset values. C<SMALL>TID</SMALL>s change after rows
|
||||
are modified or reloaded. They are used by index entries to point
|
||||
to physical rows.</P>
|
||||
|
||||
@ -917,23 +903,7 @@ length</TD></TR>
|
||||
|
||||
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
|
||||
|
||||
<H3><A name="4.15">4.15</A>) Why does my large-object operations
|
||||
get <I>"invalid large obj descriptor"</I>?</H3>
|
||||
|
||||
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
|
||||
around any use of a large object handle, that is, surrounding
|
||||
<CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
|
||||
|
||||
<P>Currently PostgreSQL enforces the rule by closing large object
|
||||
handles at transaction commit. So the first attempt to do anything
|
||||
with the handle will draw <I>invalid large obj descriptor</I>. So
|
||||
code that used to work (at least most of the time) will now
|
||||
generate that error message if you fail to use a transaction.</P>
|
||||
|
||||
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you
|
||||
may need to set <CODE>auto-commit off.</CODE></P>
|
||||
|
||||
<H3><A name="4.16">4.16</A>) How do I create a column that will
|
||||
<H3><A name="4.15">4.15</A>) How do I create a column that will
|
||||
default to the current time?</H3>
|
||||
|
||||
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
|
||||
@ -941,7 +911,7 @@ length</TD></TR>
|
||||
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
|
||||
</PRE>
|
||||
|
||||
<H3><A name="4.17">4.17</A>) How do I perform an outer join?</H3>
|
||||
<H3><A name="4.16">4.16</A>) How do I perform an outer join?</H3>
|
||||
|
||||
<P>PostgreSQL supports outer joins using the SQL standard syntax.
|
||||
Here are two examples:</P>
|
||||
@ -964,7 +934,7 @@ length</TD></TR>
|
||||
<SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
|
||||
are called <SMALL>INNER</SMALL> joins.</P>
|
||||
|
||||
<H3><A name="4.18">4.18</A>) How do I perform queries using
|
||||
<H3><A name="4.17">4.17</A>) How do I perform queries using
|
||||
multiple databases?</H3>
|
||||
|
||||
<P>There is no way to query a database other than the current one.
|
||||
@ -976,14 +946,14 @@ length</TD></TR>
|
||||
connections to different databases and merge the results on the
|
||||
client side.</P>
|
||||
|
||||
<H3><A name="4.19">4.19</A>) How do I return multiple rows or
|
||||
<H3><A name="4.18">4.18</A>) How do I return multiple rows or
|
||||
columns from a function?</H3>
|
||||
|
||||
<P>It is easy using set-returning functions,
|
||||
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
|
||||
http://techdocs.postgresql.org/guides/SetReturningFunctions</a></P>.
|
||||
|
||||
<H3><A name="4.20">4.20</A>) Why do I get "relation with OID #####
|
||||
<H3><A name="4.19">4.19</A>) Why do I get "relation with OID #####
|
||||
does not exist" errors when accessing temporary tables in PL/PgSQL
|
||||
functions?</H3>
|
||||
|
||||
@ -995,7 +965,7 @@ length</TD></TR>
|
||||
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
|
||||
will cause the query to be reparsed every time.</P>
|
||||
|
||||
<H3><A name="4.21">4.21</A>) What encryption options are available?
|
||||
<H3><A name="4.20">4.20</A>) What encryption options are available?
|
||||
</H3>
|
||||
<UL>
|
||||
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
|
||||
@ -1009,42 +979,8 @@ length</TD></TR>
|
||||
native SSL connections.)</LI>
|
||||
<LI>Database user passwords are automatically encrypted when stored in
|
||||
the system tables.</LI>
|
||||
<LI>The server can run using an encrypted file system.</LI>
|
||||
<LI>The server can also run using an encrypted file system.</LI>
|
||||
</UL>
|
||||
|
||||
<HR>
|
||||
|
||||
<H2 align="center">Extending PostgreSQL</H2>
|
||||
|
||||
<H3><A name="5.1">5.1</A>) I wrote a user-defined function. When I
|
||||
run it in <I>psql</I>, why does it dump core?</H3>
|
||||
|
||||
<P>The problem could be a number of things. Try testing your
|
||||
user-defined function in a stand-alone test program first.</P>
|
||||
|
||||
<H3><A name="5.2">5.2</A>) How can I contribute some nifty new
|
||||
types and functions to PostgreSQL?</H3>
|
||||
|
||||
<P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
|
||||
and they will eventually end up in the <I>contrib/</I>
|
||||
subdirectory.</P>
|
||||
|
||||
<H3><A name="5.3">5.3</A>) How do I write a C function to return a
|
||||
tuple?</H3>
|
||||
|
||||
<P>In versions of PostgreSQL beginning with 7.3, table-returning
|
||||
functions are fully supported in C, PL/PgSQL, and SQL. See the
|
||||
Programmer's Guide for more information. An example of a
|
||||
table-returning function defined in C can be found in
|
||||
<I>contrib/tablefunc</I>.</P>
|
||||
|
||||
<H3><A name="5.4">5.4</A>) I have changed a source file. Why does
|
||||
the recompile not see the change?</H3>
|
||||
|
||||
<P>The <I>Makefiles</I> do not have the proper dependencies for
|
||||
include files. You have to do a <I>make clean</I> and then another
|
||||
<I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
|
||||
<I>--enable-depend</I> option of <I>configure</I> to have the
|
||||
compiler compute the dependencies automatically.</P>
|
||||
</BODY>
|
||||
</HTML>
|
||||
|
||||
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Block a user