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maintained for each cache entry. A cache entry will not be freed until the matching ReleaseSysCache call has been executed. This eliminates worries about cache entries getting dropped while still in use. See my posting to pg-hackers of even date for more info.
498 lines
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HTML
498 lines
21 KiB
HTML
<HTML>
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<HEAD>
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<TITLE>PostgreSQL Developers FAQ</title>
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</HEAD>
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<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF">
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<H1>
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Developer's Frequently Asked Questions (FAQ) for PostgreSQL
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</H1>
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<P>
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Last updated: Fri Jun 9 21:54:54 EDT 2000
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<P>
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Current maintainer: Bruce Momjian (<a
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href="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</a>)<BR>
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<P>
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The most recent version of this document can be viewed at
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the postgreSQL Web site, <a
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href="http://PostgreSQL.org">http://PostgreSQL.org</a>.
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<P>
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<HR>
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<P>
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<CENTER><H2>Questions</H2></CENTER>
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<a href="#1">1</a>) What tools are available for developers?<BR>
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<a href="#2">2</a>) What books are good for developers?<BR>
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<a href="#3">3</a>) Why do we use <I>palloc</I>() and <I>pfree</I>() to allocate memory?<BR>
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<a href="#4">4</a>) Why do we use <I>Node</I> and <I>List</I> to
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make data structures?<BR>
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<a href="#5">5</a>) How do I add a feature or fix a bug?<BR>
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<a href="#6">6</a>) How do I download/update the current source tree?<BR>
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<a href="#7">7</a>) How do I test my changes?<BR>
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<a href="#7">7</a>) I just added a field to a structure. What else
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should I do?<BR>
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<a href="#8">8</a>) Why are table, column, type, function, view
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names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
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sometimes as <I>char *?</I><BR>
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<a href="#9">9</a>) How do I efficiently access information in
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tables from the backend code?<BR>
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<a href="#10">10</a>) What is elog()?<BR>
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<a href="#11">11</a>) What is configure all about?<BR>
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<a href="#12">12</a>) How do I add a new port?<BR>
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<a href="#13">13</a>) What is CommandCounterIncrement()?<BR>
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<BR>
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<HR>
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<H3><a
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name="1">1</a>) What tools are available for developers?</H3><P>
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Aside from the User documentation mentioned in the regular FAQ, there
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are several development tools available. First, all the files in the
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<I>/tools</I> directory are designed for developers.
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<PRE>
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RELEASE_CHANGES changes we have to make for each release
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SQL_keywords standard SQL'92 keywords
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backend description/flowchart of the backend directories
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ccsym find standard defines made by your compiler
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entab converts tabs to spaces, used by pgindent
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find_static finds functions that could be made static
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find_typedef get a list of typedefs in the source code
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make_ctags make vi 'tags' file in each directory
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make_diff make *.orig and diffs of source
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make_etags make emacs 'etags' files
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make_keywords.README make comparison of our keywords and SQL'92
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make_mkid make mkid ID files
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mkldexport create AIX exports file
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pgindent indents C source files
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pginclude scripts for adding/removing include files
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unused_oids in pgsql/src/include/catalog
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</PRE>
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Let me note some of these. If you point your browser at the
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<I>file:/usr/local/src/pgsql/src/tools/backend/index.html</I> directory,
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you will see few paragraphs describing the data flow, the backend
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components in a flow chart, and a description of the shared memory area.
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You can click on any flowchart box to see a description. If you then
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click on the directory name, you will be taken to the source directory,
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to browse the actual source code behind it. We also have several README
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files in some source directories to describe the function of the module.
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The browser will display these when you enter the directory also. The
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<I>tools/backend</I> directory is also contained on our web page under
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the title <I>How PostgreSQL Processes a Query.</I><P>
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Second, you really should have an editor that can handle tags, so you
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can tag a function call to see the function definition, and then tag
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inside that function to see an even lower-level function, and then back
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out twice to return to the original function. Most editors support this
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via <I>tags</I> or <I>etags</I> files.<P>
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Third, you need to get <I>id-utils</I> from:
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<pre>
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<a href="ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz">ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz</a>
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<a href="ftp://tug.org/gnu/id-utils-3.2d.tar.gz">ftp://tug.org/gnu/id-utils-3.2d.tar.gz</a>
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<a href="ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz">ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz</a>
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</pre>
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By running <I>tools/make_mkid</I>, an archive of source symbols can be
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created that can be rapidly queried like <I>grep</I> or edited. Others
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prefer <I>glimpse.</I><P>
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<I>make_diff</I> has tools to create patch diff files that can be
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applied to the distribution.<P>
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Our standard format is to indent each code level with one tab, where
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each tab is four spaces. You will need to set your editor to display
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tabs as four spaces:
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<BR>
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<PRE>
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vi in ~/.exrc:
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set tabstop=4
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set sw=4
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more:
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more -x4
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less:
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less -x4
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emacs:
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M-x set-variable tab-width
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or
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; Cmd to set tab stops &etc for working with PostgreSQL code
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(c-add-style "pgsql"
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'("bsd"
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(indent-tabs-mode . t)
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(c-basic-offset . 4)
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(tab-width . 4)
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(c-offsets-alist .
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((case-label . +))))
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t) ; t = set this mode on
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and add this to your autoload list (modify file path in macro):
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(setq auto-mode-alist
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(cons '("\\`/usr/local/src/pgsql/.*\\.[chyl]\\'" . pgsql-c-mode)
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auto-mode-alist))
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or
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/*
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* Local variables:
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* tab-width: 4
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* c-indent-level: 4
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* c-basic-offset: 4
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* End:
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*/
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</PRE>
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<BR>
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<I>pgindent</I> will the format code by specifying
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flags to your operating system's utility <I>indent.</I><P>
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<I>pgindent</I> is run on all source files just before each beta test
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period. It auto-formats all source files to make them consistent.
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Comment blocks that need specific line breaks should be formatted as
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<I>block comments,</I> where the comment starts as
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<CODE>/*------</CODE>. These comments will not be reformatted in any
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way.
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<I>pginclude</I> contains scripts used to add needed #include's to
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include files, and removed unneeded #include's.
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When adding system types, you will need to assign oids to them.
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There is also a script called <I>unused_oids</I> in
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<I>pgsql/src/include/catalog</I> that shows the unused oids.
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<H3><a name="2">2</a>) What books are good for developers?</H3><P>
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I have four good books, <I>An Introduction to Database Systems,</I> by
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C.J. Date, Addison, Wesley, <I>A Guide to the SQL Standard,</I> by C.J.
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Date, et. al, Addison, Wesley, <I>Fundamentals of Database Systems,</I>
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by Elmasri and Navathe, and <I>Transaction Processing,</I> by Jim Gray,
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Morgan, Kaufmann<P>
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There is also a database performance site, with a handbook on-line
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written by Jim Gray at <A
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HREF="http://www.benchmarkresources.com">http://www.benchmarkresources.com.</A>
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<H3><a name="3">3</a>) Why do we use <I>palloc</I>() and <I>pfree</I>()
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to allocate memory?</H3><P>
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<I>palloc()</I> and <I>pfree()</I> are used in place of malloc() and
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free() because we automatically free all memory allocated when a
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transaction completes. This makes it easier to make sure we free memory
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that gets allocated in one place, but only freed much later. There are
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several contexts that memory can be allocated in, and this controls when
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the allocated memory is automatically freed by the backend.<P>
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<H3><a name="4">4</a>) Why do we use <I>Node</I> and <I>List</I> to
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make data structures?</H3><P>
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We do this because this allows a consistent way to pass data inside the
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backend in a flexible way. Every node has a <I>NodeTag</I> which
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specifies what type of data is inside the Node. <I>Lists</I> are groups
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of <I>Nodes chained together as a forward-linked list.</I><P>
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Here are some of the <I>List</I> manipulation commands:
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<BLOCKQUOTE>
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<DL>
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<DT>lfirst(i)
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<DD>return the data at list element <I>i.</I>
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<DT>lnext(i)
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<DD>return the next list element after <I>i.</I>
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<DT>foreach(i, list)
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<DD>loop through <I>list,</I> assigning each list element to <I>i.</I>
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It is important to note that <I>i</I> is a List *, not the data in the
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<I>List</I> element. You need to use <I>lfirst(i)</I> to get at the data.
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Here is a typical code snipped that loops through a List containing
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<I>Var *'s</I> and processes each one:
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<PRE>
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<CODE>
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List *i, *list;
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foreach(i, list)
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{
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Var *var = lfirst(i);
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/* process var here */
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}
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</CODE>
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</PRE>
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<DT>lcons(node, list)
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<DD>add <I>node</I> to the front of <I>list,</I> or create a new list with
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<I>node</I> if <I>list</I> is <I>NIL.</I>
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<DT>lappend(list, node)
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<DD>add <I>node</I> to the end of <I>list.</I> This is more expensive
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that lcons.
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<DT>nconc(list1, list2)
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<DD>Concat <I>list2</I> on to the end of <I>list1.</I>
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<DT>length(list)
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<DD>return the length of the <I>list.</I>
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<DT>nth(i, list)
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<DD>return the <I>i</I>'th element in <I>list.</I>
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<DT>lconsi, ...
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<DD>There are integer versions of these: <I>lconsi, lappendi, nthi.</I>
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<I>List's</I> containing integers instead of Node pointers are used to
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hold list of relation object id's and other integer quantities.
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</DL>
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</BLOCKQUOTE>
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You can print nodes easily inside <I>gdb.</I> First, to disable
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output truncation when you use the gdb <I>print</I> command:
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<PRE>
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<CODE>
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(gdb) set print elements 0
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</CODE>
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</PRE>
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Instead of printing values in gdb format, you can use the next two
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commands to print out List, Node, and structure contents in a verbose
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format that is easier to understand. List's are unrolled into nodes,
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and nodes are printed in detail. The first prints in a short format,
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and the second in a long format:
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<PRE>
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<CODE>
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(gdb) call print(any_pointer)
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(gdb) call pprint(any_pointer)
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</CODE>
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</PRE>
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The output appears in the postmaster log file, or on your screen if you
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are running a backend directly without a postmaster.
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<P>
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<H3><a name="5">5</a>) How do I add a feature or fix a bug?</H3><P>
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The source code is over 250,000 lines. Many problems/features are
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isolated to one specific area of the code. Others require knowledge of
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much of the source. If you are confused about where to start, ask the
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hackers list, and they will be glad to assess the complexity and give
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pointers on where to start.<P>
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Another thing to keep in mind is that many fixes and features can be
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added with surprisingly little code. I often start by adding code, then
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looking at other areas in the code where similar things are done, and by
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the time I am finished, the patch is quite small and compact.<P>
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When adding code, keep in mind that it should use the existing
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facilities in the source, for performance reasons and for simplicity.
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Often a review of existing code doing similar things is helpful.<P>
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<H3><a name="6">6</a>) How do I download/update the current source
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tree?</H3><P>
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There are several ways to obtain the source tree. Occasional developers
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can just get the most recent source tree snapshot from
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ftp.postgresql.org. For regular developers, you can use CVS. CVS
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allows you to download the source tree, then occasionally update your
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copy of the source tree with any new changes. Using CVS, you don't have
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to download the entire source each time, only the changed files.
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Anonymous CVS does not allows developers to update the remote source
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tree, though privileged developers can do this. There is a CVS FAQ on
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our web site that describes how to use remote CVS. You can also use
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CVSup, which has similarly functionality, and is available from
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ftp.postgresql.org.<P>
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To update the source tree, there are two ways. You can generate a patch
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against your current source tree, perhaps using the make_diff tools
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mentioned above, and send them to the patches list. They will be
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reviewed, and applied in a timely manner. If the patch is major, and we
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are in beta testing, the developers may wait for the final release
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before applying your patches.<P>
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For hard-core developers, Marc(scrappy@postgresql.org) will give you a
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Unix shell account on postgresql.org, so you can use CVS to update the
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main source tree, or you can ftp your files into your account, patch,
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and cvs install the changes directly into the source tree. <P>
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<H3><a name="6">6</a>) How do I test my changes?</H3><P>
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First, use <I>psql</I> to make sure it is working as you expect. Then
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run <I>src/test/regress</I> and get the output of
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<I>src/test/regress/checkresults</I> with and without your changes, to
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see that your patch does not change the regression test in unexpected
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ways. This practice has saved me many times. The regression tests test
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the code in ways I would never do, and has caught many bugs in my
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patches. By finding the problems now, you save yourself a lot of
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debugging later when things are broken, and you can't figure out when it
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happened.<P>
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<H3><a name="7">7</a>) I just added a field to a structure. What else
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should I do?</H3><P>
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The structures passing around from the parser, rewrite, optimizer, and
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executor require quite a bit of support. Most structures have support
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routines in <I>src/backend/nodes</I> used to create, copy, read, and output
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those structures. Make sure you add support for your new field to these
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files. Find any other places the structure may need code for your new
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field. <I>mkid</I> is helpful with this (see above).<P>
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<H3><a name="8">8</a>) Why are table, column, type, function, view
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names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
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sometimes as <I>char *?</I></H3><P>
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Table, column, type, function, and view names are stored in system
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tables in columns of type <I>Name.</I> Name is a fixed-length,
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null-terminated type of <I>NAMEDATALEN</I> bytes. (The default value
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for NAMEDATALEN is 32 bytes.)
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<PRE><CODE>
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typedef struct nameData
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{
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char data[NAMEDATALEN];
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} NameData;
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typedef NameData *Name;
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</CODE></PRE>
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Table, column, type, function, and view names that come into the
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backend via user queries are stored as variable-length, null-terminated
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character strings.<P>
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Many functions are called with both types of names, ie. <I>heap_open().</I>
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Because the Name type is null-terminated, it is safe to pass it to a
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function expecting a char *. Because there are many cases where on-disk
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names(Name) are compared to user-supplied names(char *), there are many
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cases where Name and char * are used interchangeably.<P>
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<H3><a name="9">9</a>) How do I efficiently access information in
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tables from the backend code?</H3><P>
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You first need to find the tuples(rows) you are interested in. There
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are two ways. First, <I>SearchSysCache()</I> and related functions
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allow you to query the system catalogs. This is the preferred way to
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access system tables, because the first call to the cache loads the
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needed rows, and future requests can return the results without
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accessing the base table. The caches use system table indexes
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to look up tuples. A list of available caches is located in
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<I>src/backend/utils/cache/syscache.c.</I>
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<I>src/backend/utils/cache/lsyscache.c</I> contains many column-specific
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cache lookup functions.<P>
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The rows returned are cache-owned versions of the heap rows. Therefore, you
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must not modify or delete the tuple returned by <I>SearchSysCache()</I>. What
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you <I>should</I> do is release it with <I>ReleaseSysCache()</I> when you are
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done using it; this informs the cache that it can discard that tuple if
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necessary. If you neglect to call <I>ReleaseSysCache()</I>, then the cache
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entry will remain locked in the cache until end of transaction, which is
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tolerable but not very desirable.<P>
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If you can't use the system cache, you will need to retrieve the data
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directly from the heap table, using the buffer cache that is shared by
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all backends. The backend automatically takes care of loading the rows
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into the buffer cache.<P>
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Open the table with <I>heap_open().</I> You can then start a table scan
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with <I>heap_beginscan(),</I> then use <I>heap_getnext()</I> and
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continue as long as <I>HeapTupleIsValid()</I> returns true. Then do a
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<I>heap_endscan().</I> <I>Keys</I> can be assigned to the <I>scan.</I>
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No indexes are used, so all rows are going to be compared to the keys,
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and only the valid rows returned.<P>
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You can also use <I>heap_fetch()</I> to fetch rows by block
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number/offset. While scans automatically lock/unlock rows from the
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buffer cache, with <I>heap_fetch(),</I> you must pass a <I>Buffer</I>
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pointer, and <I>ReleaseBuffer()</I> it when completed.<P>
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Once you have the row, you can get data that is common to all tuples,
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like <I>t_self</I> and <I>t_oid,</I> by merely accessing the
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<I>HeapTuple</I> structure entries.
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If you need a table-specific column, you should take the HeapTuple
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pointer, and use the <I>GETSTRUCT()</I> macro to access the
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table-specific start of the tuple. You then cast the pointer as a
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<I>Form_pg_proc</I> pointer if you are accessing the pg_proc table, or
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<I>Form_pg_type</I> if you are accessing pg_type. You can then access
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the columns by using a structure pointer:
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<PRE>
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<CODE>
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((Form_pg_class) GETSTRUCT(tuple))->relnatts
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</CODE>
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</PRE>
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You must not directly change <I>live</I> tuples in this way. The best
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way is to use <I>heap_modifytuple()</I> and pass it your original
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tuple, and the values you want changed. It returns a palloc'ed
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tuple, which you pass to <I>heap_replace().</I>
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You can delete tuples by passing the tuple's <I>t_self</I> to
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<I>heap_destroy().</I> You use <I>t_self</I> for <I>heap_update()</I> too.
|
|
|
|
Remember, tuples can be either system cache copies, which may go away after
|
|
you call <I>ReleaseSysCache()</I>, or read directly from disk buffers, which
|
|
go away when you <I>heap_getnext()</I>, <I>heap_endscan</I>, or
|
|
<I>ReleaseBuffer()</I>, in the <I>heap_fetch()</I> case. Or it may be a
|
|
palloc'ed tuple, that you must <I>pfree()</I> when finished.
|
|
|
|
<H3><a name="10">10</a>) What is elog()?</H3><P>
|
|
|
|
<I>elog()</I> is used to send messages to the front-end, and optionally
|
|
terminate the current query being processed. The first parameter is an
|
|
elog level of <I>NOTICE,</I> <I>DEBUG,</I> <I>ERROR,</I> or
|
|
<I>FATAL.</I>
|
|
|
|
<I>NOTICE</I> prints on the user's terminal and the postmaster logs.
|
|
<I>DEBUG</I> prints only in the postmaster logs. <I>ERROR</I> prints in
|
|
both places, and terminates the current query, never returning from the call.
|
|
<I>FATAL</I> terminates the backend process.
|
|
|
|
The remaining parameters of <I>elog</I> are a <I>printf</I>-style set of
|
|
parameters to print.
|
|
|
|
<H3><a name="11">11</a>) What is configure all about?</H3><P>
|
|
|
|
The files <I>configure</I> and <I>configure.in</I> are part of the
|
|
GNU <I>autoconf</I> package. Configure allows us to test for various
|
|
capabilities of the OS, and to set variables that can then be tested in
|
|
C programs and Makefiles. Autoconf is installed on the PostgreSQL main
|
|
server. To add options to configure, edit <I>configure.in,</I> and then
|
|
run <I>autoconf</I> to generate <I>configure.</I><P>
|
|
|
|
When <I>configure</I> is run by the user, it tests various OS
|
|
capabilities, stores those in <I>config.status</I> and
|
|
<I>config.cache,</I> and modifies a list of <I>*.in</I> files. For
|
|
example, if there exists a <I>Makefile.in,</I> configure generates a
|
|
<I>Makefile</I> that contains substitutions for all @var@ parameters
|
|
found by configure.<P>
|
|
|
|
When you need to edit files, make sure you don't waste time modifying
|
|
files generated by <I>configure.</I> Edit the <I>*.in</I> file, and
|
|
re-run <I>configure</I> to recreate the needed file. If you run <I>make
|
|
distclean</I> from the top-level source directory, all files derived by
|
|
configure are removed, so you see only the file contained in the source
|
|
distribution.<P>
|
|
|
|
<H3><a name="12">12</a>) How do I add a new port?</H3><P>
|
|
|
|
There are a variety of places that need to be modified to add a new
|
|
port. First, start in the <I>src/template</I> directory. Add an
|
|
appropriate entry for your OS. Also, use <I>src/config.guess</I> to add
|
|
your OS to <I>src/template/.similar.</I> You shouldn't match the OS
|
|
version exactly. The <I>configure</I> test will look for an exact OS
|
|
version number, and if not found, find a match without version number.
|
|
Edit <I>src/configure.in</I> to add your new OS. (See configure item
|
|
above.) You will need to run autoconf, or patch <I>src/configure</I>
|
|
too.<P>
|
|
|
|
Then, check <I>src/include/port</I> and add your new OS file, with
|
|
appropriate values. Hopefully, there is already locking code in
|
|
<I>src/include/storage/s_lock.h</I> for your CPU. There is also a
|
|
<I>src/makefiles</I> directory for port-specific Makefile handling.
|
|
There is a <I>backend/port</I> directory if you need special files for
|
|
your OS.<P>
|
|
|
|
<H3><a name="13">13</a>) What is CommandCounterIncrement()?</H3><P>
|
|
|
|
Normally, transactions can not see the rows they modify. This allows <CODE>
|
|
UPDATE foo SET x = x + 1</CODE> to work correctly.
|
|
<P>
|
|
|
|
However, there are cases where a transactions needs to see rows affected
|
|
in previous parts of the transaction. This is accomplished using a
|
|
Command Counter. Incrementing the counter allows transactions to be
|
|
broken into pieces so each piece can see rows modified by previous
|
|
pieces. <I>CommandCounterIncrement()</I> increments the Command
|
|
Counter, creating a new part of the transaction. <P>
|
|
|
|
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