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<H1>Developer's Frequently Asked Questions (FAQ) for
PostgreSQL</H1>
<P>Last updated: Tue Feb 10 10:16:31 EST 2004</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
</P>
<P>The most recent version of this document can be viewed at <A href=
"http://www.PostgreSQL.org/docs/faqs/FAQ_DEV.html">http://www.PostgreSQL.org/docs/faqs/FAQ_DEV.html</A>.</P>
<HR>
<BR>
<CENTER>
<H2>General Questions</H2>
</CENTER>
<A href="#1.1">1.1</A>) How do I get involved in PostgreSQL
development?<BR>
<A href="#1.2">1.2</A>) How do I add a feature or fix a bug?<BR>
<A href="#1.3">1.3</A>) How do I download/update the current source
tree?<BR>
<A href="#1.4">1.4</A>) How do I test my changes?<BR>
<A href="#1.5">1.5</A>) What tools are available for developers?<BR>
<A href="#1.6">1.6</A>) What books are good for developers?<BR>
<A href="#1.7">1.7</A>) What is configure all about?<BR>
<A href="#1.8">1.8</A>) How do I add a new port?<BR>
<A href="#1.9">1.9</A>) Why don't you use threads/raw
devices/async-I/O, <insert your favorite wizz-bang feature
here>?<BR>
<A href="#1.10">1.10</A>) How are RPM's packaged?<BR>
<A href="#1.11">1.11</A>) How are CVS branches handled?<BR>
<A href="#1.12">1.12</A>) Where can I get a copy of the SQL
standards?<BR>
<CENTER>
<H2>Technical Questions</H2>
</CENTER>
<A href="#2.1">2.1</A>) How do I efficiently access information in
tables from the backend code?<BR>
<A href="#2.2">2.2</A>) Why are table, column, type, function, view
names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
sometimes as <I>char *?</I><BR>
<A href="#2.3">2.3</A>) Why do we use <I>Node</I> and <I>List</I> to
make data structures?<BR>
<A href="#2.4">2.4</A>) I just added a field to a structure. What else
should I do?<BR>
<A href="#2.5">2.5</A>) Why do we use <I>palloc</I>() and
<I>pfree</I>() to allocate memory?<BR>
<A href="#2.6">2.6</A>) What is ereport()?<BR>
<A href="#2.7">2.7</A>) What is CommandCounterIncrement()?<BR>
<BR>
<HR>
<CENTER>
<H2>General Questions</H2> </CENTER>
<H3><A name="1.1">1.1</A>) How go I get involved in PostgreSQL
development?</H3>
<P>This was written by Lamar Owen:</P>
<P>2001-06-22</P>
<B>What open source development process is used by the PostgreSQL
team?</B>
<P>Read HACKERS for six months (or a full release cycle, whichever
is longer). Really. HACKERS _is_the process. The process is not
well documented (AFAIK -- it may be somewhere that I am not aware
of) -- and it changes continually.</P>
<B>What development environment (OS, system, compilers, etc) is
required to develop code?</B>
<P><A href="http://developer.postgresql.org">Developers Corner</A> on the
website has links to this information. The distribution tarball
itself includes all the extra tools and documents that go beyond a
good Unix-like development environment. In general, a modern unix
with a modern gcc, GNU make or equivalent, autoconf (of a
particular version), and good working knowledge of those tools are
required.</P>
<B>What areas need support?</B>
<P>The TODO list.</P>
<P>You've made the first step, by finding and subscribing to
HACKERS. Once you find an area to look at in the TODO, and have
read the documentation on the internals, etc, then you check out a
current CVS,write what you are going to write (keeping your CVS
checkout up to date in the process), and make up a patch (as a
context diff only) and send to the PATCHES list, prefereably.</P>
<P>Discussion on the patch typically happens here. If the patch
adds a major feature, it would be a good idea to talk about it
first on the HACKERS list, in order to increase the chances of it
being accepted, as well as toavoid duplication of effort. Note that
experienced developers with a proven track record usually get the
big jobs -- for more than one reason. Also note that PostgreSQL is
highly portable -- nonportable code will likely be dismissed out of
hand.</P>
<P>Once your contributions get accepted, things move from there.
Typically, you would be added as a developer on the list on the
website when one of the other developers recommends it. Membership
on the steering committee is by invitation only, by the other
steering committee members, from what I have gathered watching
froma distance.</P>
<P>I make these statements from having watched the process for over
two years.</P>
<P>To see a good example of how one goes about this, search the
archives for the name 'Tom Lane' and see what his first post
consisted of, and where he took things. In particular, note that
this hasn't been _that_ long ago -- and his bugfixing and general
deep knowledge with this codebase is legendary. Take a few days to
read after him. And pay special attention to both the sheer
quantity as well as the painstaking quality of his work. Both are
in high demand.</P>
<H3><A name="1.2">1.2</A>) How do I add a feature or fix a bug?</H3>
<P>The source code is over 350,000 lines. Many fixes/features are isolated to one specific area of the code. Others require
knowledge of much of the source. If you are confused about where to
start, ask the hackers list, and they will be glad to assess the
complexity and give pointers on where to start.</P>
<P>Another thing to keep in mind is that many fixes and features
can be added with surprisingly little code. I often start by adding
code, then looking at other areas in the code where similar things
are done, and by the time I am finished, the patch is quite small
and compact.</P>
<P>When adding code, keep in mind that it should use the existing
facilities in the source, for performance reasons and for
simplicity. Often a review of existing code doing similar things is
helpful.</P>
<P>The usual process for source additions is:
<UL>
<LI>Review the TODO list.</LI>
<LI>Discuss hackers the desirability of the fix/feature.</LI>
<LI>How should it behave in complex circumstances?</LI>
<LI>How should it be implemented?</LI>
<LI>Submit the patch to the patches list.</LI>
<LI>Answer email questions.</LI>
<LI>Wait for the patch to be applied.</LI>
</UL></P>
<H3><A name="1.3">1.3</A>) How do I download/update the current source
tree?</H3>
<P>There are several ways to obtain the source tree. Occasional
developers can just get the most recent source tree snapshot from
ftp.postgresql.org. For regular developers, you can use CVS. CVS
allows you to download the source tree, then occasionally update
your copy of the source tree with any new changes. Using CVS, you
don't have to download the entire source each time, only the
changed files. Anonymous CVS does not allows developers to update
the remote source tree, though privileged developers can do this.
There is a CVS FAQ on our web site that describes how to use remote
CVS. You can also use CVSup, which has similarly functionality, and
is available from ftp.postgresql.org.</P>
<P>To update the source tree, there are two ways. You can generate
a patch against your current source tree, perhaps using the
make_diff tools mentioned above, and send them to the patches list.
They will be reviewed, and applied in a timely manner. If the patch
is major, and we are in beta testing, the developers may wait for
the final release before applying your patches.</P>
<P>For hard-core developers, Marc(scrappy@postgresql.org) will give
you a Unix shell account on postgresql.org, so you can use CVS to
update the main source tree, or you can ftp your files into your
account, patch, and cvs install the changes directly into the
source tree.</P>
<H3><A name="1.4">1.4</A>) How do I test my changes?</H3>
<P>First, use <I>psql</I> to make sure it is working as you expect.
Then run <I>src/test/regress</I> and get the output of
<I>src/test/regress/checkresults</I> with and without your changes,
to see that your patch does not change the regression test in
unexpected ways. This practice has saved me many times. The
regression tests test the code in ways I would never do, and has
caught many bugs in my patches. By finding the problems now, you
save yourself a lot of debugging later when things are broken, and
you can't figure out when it happened.</P>
<H3><A name="1.5">1.5</A>) What tools are available for
developers?</H3>
<P>Aside from the User documentation mentioned in the regular FAQ, there are several development tools available. First, all the files
in the <I>/tools</I> directory are designed for developers.</P>
<PRE>
RELEASE_CHANGES changes we have to make for each release
SQL_keywords standard SQL'92 keywords
backend description/flowchart of the backend directories
ccsym find standard defines made by your compiler
entab converts tabs to spaces, used by pgindent
find_static finds functions that could be made static
find_typedef finds typedefs in the source code
find_badmacros finds macros that use braces incorrectly
make_ctags make vi 'tags' file in each directory
make_diff make *.orig and diffs of source
make_etags make emacs 'etags' files
make_keywords make comparison of our keywords and SQL'92
make_mkid make mkid ID files
mkldexport create AIX exports file
pgindent indents C source files
pgjindent indents Java source files
pginclude scripts for adding/removing include files
unused_oids in pgsql/src/include/catalog
</PRE>
Let me note some of these. If you point your browser at the
<I>file:/usr/local/src/pgsql/src/tools/backend/index.html</I>
directory, you will see few paragraphs describing the data flow,
the backend components in a flow chart, and a description of the
shared memory area. You can click on any flowchart box to see a
description. If you then click on the directory name, you will be
taken to the source directory, to browse the actual source code
behind it. We also have several README files in some source
directories to describe the function of the module. The browser
will display these when you enter the directory also. The
<I>tools/backend</I> directory is also contained on our web page
under the title <I>How PostgreSQL Processes a Query.</I>
<P>Second, you really should have an editor that can handle tags,
so you can tag a function call to see the function definition, and
then tag inside that function to see an even lower-level function,
and then back out twice to return to the original function. Most
editors support this via <I>tags</I> or <I>etags</I> files.</P>
<P>Third, you need to get <I>id-utils</I> from:</P>
<PRE>
<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>
<A href=
"ftp://tug.org/gnu/id-utils-3.2d.tar.gz">ftp://tug.org/gnu/id-utils-3.2d.tar.gz</A>
<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>
</PRE>
By running <I>tools/make_mkid</I>, an archive of source symbols can
be created that can be rapidly queried like <I>grep</I> or edited.
Others prefer <I>glimpse.</I>
<P><I>make_diff</I> has tools to create patch diff files that can
be applied to the distribution. This produces context diffs, which
is our preferred format.</P>
<P>Our standard format is to indent each code level with one tab,
where each tab is four spaces. You will need to set your editor to
display tabs as four spaces:<BR>
</P>
<PRE>
vi in ~/.exrc:
set tabstop=4
set sw=4
more:
more -x4
less:
less -x4 emacs:
M-x set-variable tab-width
or
(c-add-style "pgsql"
'("bsd"
(indent-tabs-mode . t)
(c-basic-offset . 4)
(tab-width . 4)
(c-offsets-alist .
((case-label . +)))
)
nil ) ; t = set this style, nil = don't
(defun pgsql-c-mode ()
(c-mode)
(c-set-style "pgsql")
)
and add this to your autoload list (modify file path in macro):
(setq auto-mode-alist
(cons '("\\`/home/andrew/pgsql/.*\\.[chyl]\\'" . pgsql-c-mode)
auto-mode-alist))
or
/*
* Local variables:
* tab-width: 4
* c-indent-level: 4
* c-basic-offset: 4
* End:
*/
</PRE>
<BR>
<I>pgindent</I> will the format code by specifying flags to your
operating system's utility <I>indent.</I> This
<A HREF="http://ezine.daemonnews.org/200112/single_coding_style.html">
article</A> describes the value of a constent coding style.
<P><I>pgindent</I> is run on all source files just before each beta
test period. It auto-formats all source files to make them
consistent. Comment blocks that need specific line breaks should be
formatted as <I>block comments,</I> where the comment starts as
<CODE>/*------</CODE>. These comments will not be reformatted in
any way.</P>
<P><I>pginclude</I> contains scripts used to add needed
<CODE>#include</CODE>'s to include files, and removed unneeded
<CODE>#include</CODE>'s.</P>
<P>When adding system types, you will need to assign oids to them.
There is also a script called <I>unused_oids</I> in
<I>pgsql/src/include/catalog</I> that shows the unused oids.</P>
<H3><A name="1.6">1.6</A>) What books are good for developers?</H3>
<P>I have four good books, <I>An Introduction to Database
Systems,</I> by C.J. Date, Addison, Wesley, <I>A Guide to the SQL
Standard,</I> by C.J. Date, et. al, Addison, Wesley,
<I>Fundamentals of Database Systems,</I> by Elmasri and Navathe,
and <I>Transaction Processing,</I> by Jim Gray, Morgan,
Kaufmann</P>
<P>There is also a database performance site, with a handbook
on-line written by Jim Gray at <A href=
"http://www.benchmarkresources.com">http://www.benchmarkresources.com.</A></P>
<H3><A name="1.7">1.7</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>
<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>
<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="1.8">1.8</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>
<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="1.9">1.9</A>) Why don't you use threads/raw
devices/async-I/O, <insert your favorite wizz-bang feature
here>?</H3>
<P>There is always a temptation to use the newest operating system
features as soon as they arrive. We resist that temptation.</P>
<P>First, we support 15+ operating systems, so any new feature has
to be well established before we will consider it. Second, most new
<I>wizz-bang</I> features don't provide <I>dramatic</I>
improvements. Third, they usually have some downside, such as
decreased reliability or additional code required. Therefore, we
don't rush to use new features but rather wait for the feature to be
established, then ask for testing to show that a measurable
improvement is possible.</P>
<P>As an example, threads are not currently used in the backend code
because:</P>
<UL>
<LI>Historically, threads were unsupported and buggy.</LI>
<LI>An error in one backend can corrupt other backends.</LI>
<LI>Speed improvements using threads are small compared to the
remaining backend startup time.</LI>
<LI>The backend code would be more complex.</LI>
</UL>
<P>So, we are not ignorant of new features. It is just that
we are cautious about their adoption. The TODO list often
contains links to discussions showing our reasoning in
these areas.</P>
<H3><A name="1.10">1.10</A>) How are RPM's packaged?</H3>
<P>This was written by Lamar Owen:</P>
<P>2001-05-03</P>
<P>As to how the RPMs are built -- to answer that question sanely
requires me to know how much experience you have with the whole RPM
paradigm. 'How is the RPM built?' is a multifaceted question. The
obvious simple answer is that I maintain:</P>
<OL>
<LI>A set of patches to make certain portions of the source tree
'behave' in the different environment of the RPMset;</LI>
<LI>The initscript;</LI>
<LI>Any other ancilliary scripts and files;</LI>
<LI>A README.rpm-dist document that tries to adequately document
both the differences between the RPM build and the WHY of the
differences, as well as useful RPM environment operations (like,
using syslog, upgrading, getting postmaster to start at OS boot,
etc);</LI>
<LI>The spec file that throws it all together. This is not a
trivial undertaking in a package of this size.</LI>
</OL>
<P>I then download and build on as many different canonical
distributions as I can -- currently I am able to build on Red Hat
6.2, 7.0, and 7.1 on my personal hardware. Occasionally I receive
opportunity from certain commercial enterprises such as Great
Bridge and PostgreSQL, Inc. to build on other distributions.</P>
<P>I test the build by installing the resulting packages and
running the regression tests. Once the build passes these tests, I
upload to the postgresql.org ftp server and make a release
announcement. I am also responsible for maintaining the RPM
download area on the ftp site.</P>
<P>You'll notice I said 'canonical' distributions above. That
simply means that the machine is as stock 'out of the box' as
practical -- that is, everything (except select few programs) on
these boxen are installed by RPM; only official Red Hat released
RPMs are used (except in unusual circumstances involving software
that will not alter the build -- for example, installing a newer
non-RedHat version of the Dia diagramming package is OK --
installing Python 2.1 on the box that has Python 1.5.2 installed is
not, as that alters the PostgreSQL build). The RPM as uploaded is
built to as close to out-of-the-box pristine as is possible. Only
the standard released 'official to that release' compiler is used
-- and only the standard official kernel is used as well.</P>
<P>For a time I built on Mandrake for RedHat consumption -- no
more. Nonstandard RPM building systems are worse than useless.
Which is not to say that Mandrake is useless! By no means is
Mandrake useless -- unless you are building Red Hat RPMs -- and Red
Hat is useless if you're trying to build Mandrake or SuSE RPMs, for
that matter. But I would be foolish to use 'Lamar Owen's Super
Special RPM Blend Distro 0.1.2' to build for public consumption!
:-)</P>
<P>I _do_ attempt to make the _source_ RPM compatible with as many distributions as possible -- however, since I have limited
resources (as a volunteer RPM maintainer) I am limited as to the
amount of testing said build will get on other distributions,
architectures, or systems.</P>
<P>And, while I understand people's desire to immediately upgrade
to the newest version, realize that I do this as a side interest --
I have a regular, full-time job as a broadcast
engineer/webmaster/sysadmin/Technical Director which occasionally
prevents me from making timely RPM releases. This happened during
the early part of the 7.1 beta cycle -- but I believe I was pretty
much on the ball for the Release Candidates and the final
release.</P>
<P>I am working towards a more open RPM distribution -- I would
dearly love to more fully document the process and put everything
into CVS -- once I figure out how I want to represent things such
as the spec file in a CVS form. It makes no sense to maintain a
changelog, for instance, in the spec file in CVS when CVS does a
better job of changelogs -- I will need to write a tool to generate
a real spec file from a CVS spec-source file that would add version
numbers, changelog entries, etc to the result before building the
RPM. IOW, I need to rethink the process -- and then go through the
motions of putting my long RPM history into CVS one version at a
time so that version history information isn't lost.</P>
<P>As to why all these files aren't part of the source tree, well,
unless there was a large cry for it to happen, I don't believe it
should. PostgreSQL is very platform-agnostic -- and I like that.
Including the RPM stuff as part of the Official Tarball (TM) would,
IMHO, slant that agnostic stance in a negative way. But maybe I'm
too sensitive to that. I'm not opposed to doing that if that is the
consensus of the core group -- and that would be a sneaky way to
get the stuff into CVS :-). But if the core group isn't thrilled
with the idea (and my instinct says they're not likely to be), I am
opposed to the idea -- not to keep the stuff to myself, but to not
hinder the platform-neutral stance. IMHO, of course.</P>
<P>Of course, there are many projects that DO include all the files
necessary to build RPMs from their Official Tarball (TM).</P>
<H3><A name="1.11">1.11</A>) How are CVS branches managed?</H3>
<P>This was written by Tom Lane:</P>
<P>2001-05-07</P>
<P>If you just do basic "cvs checkout", "cvs update", "cvs commit",
then you'll always be dealing with the HEAD version of the files in
CVS. That's what you want for development, but if you need to patch
past stable releases then you have to be able to access and update
the "branch" portions of our CVS repository. We normally fork off a
branch for a stable release just before starting the development
cycle for the next release.</P>
<P>The first thing you have to know is the branch name for the
branch you are interested in getting at. To do this, look at some
long-lived file, say the top-level HISTORY file, with "cvs status
-v" to see what the branch names are. (Thanks to Ian Lance Taylor
for pointing out that this is the easiest way to do it.) Typical
branch names are:</P>
<PRE>
REL7_1_STABLE
REL7_0_PATCHES
REL6_5_PATCHES
</PRE>
<P>OK, so how do you do work on a branch? By far the best way is to
create a separate checkout tree for the branch and do your work in
that. Not only is that the easiest way to deal with CVS, but you really need to have the whole past tree available anyway to test
your work. (And you *better* test your work. Never forget that
dot-releases tend to go out with very little beta testing --- so
whenever you commit an update to a stable branch, you'd better be
doubly sure that it's correct.)</P>
<P>Normally, to checkout the head branch, you just cd to the place
you want to contain the toplevel "pgsql" directory and say</P>
<PRE>
cvs ... checkout pgsql
</PRE>
<P>To get a past branch, you cd to whereever you want it and
say</P>
<PRE>
cvs ... checkout -r BRANCHNAME pgsql
</PRE>
<P>For example, just a couple days ago I did</P>
<PRE>
mkdir ~postgres/REL7_1
cd ~postgres/REL7_1
cvs ... checkout -r REL7_1_STABLE pgsql
</PRE>
<P>and now I have a maintenance copy of 7.1.*.</P>
<P>When you've done a checkout in this way, the branch name is
"sticky": CVS automatically knows that this directory tree is for
the branch, and whenever you do "cvs update" or "cvs commit" in
this tree, you'll fetch or store the latest version in the branch,
not the head version. Easy as can be.</P>
<P>So, if you have a patch that needs to apply to both the head and
a recent stable branch, you have to make the edits and do the
commit twice, once in your development tree and once in your stable
branch tree. This is kind of a pain, which is why we don't normally
fork the tree right away after a major release --- we wait for a
dot-release or two, so that we won't have to double-patch the first
wave of fixes.</P>
<H3><A name="1.12">1.12</A>) Where can I get a copy of the SQL
standards?</H3>
<P>There are two pertinent standards, SQL92 and SQL99. These
standards are endorsed by ANSI and ISO. A draft of the SQL92
standard is available at <a
href="http://www.contrib.andrew.cmu.edu/~shadow/">
http://www.contrib.andrew.cmu.edu/~shadow/</a>. The SQL99 standard
must be purchased from ANSI at <a
href="http://webstore.ansi.org/ansidocstore/default.asp">
http://webstore.ansi.org/ansidocstore/default.asp</a>. The main
standards documents are ANSI X3.135-1992 for SQL92 and ANSI/ISO/IEC
9075-2-1999 for SQL99. The SQL 200X standards are at <a href=
"ftp://sqlstandards.org/SC32/WG3/Progression_Documents/FCD">
ftp://sqlstandards.org/SC32/WG3/Progression_Documents/FCD</A></P>
<P>A summary of these standards is at <a
href="http://dbs.uni-leipzig.de/en/lokal/standards.pdf">
http://dbs.uni-leipzig.de/en/lokal/standards.pdf</a> and <a
href="http://db.konkuk.ac.kr/present/SQL3.pdf">
http://db.konkuk.ac.kr/present/SQL3.pdf</a>.</P>
<CENTER>
<H2>Technical Questions</H2>
</CENTER>
<H3><A name="2.1">2.1</A>) How do I efficiently access information in
tables from the backend code?</H3>
<P>You first need to find the tuples(rows) you are interested in.
There are two ways. First, <I>SearchSysCache()</I> and related
functions allow you to query the system catalogs. This is the
preferred way to access system tables, because the first call to
the cache loads the needed rows, and future requests can return the
results without accessing the base table. The caches use system
table indexes to look up tuples. A list of available caches is
located in <I>src/backend/utils/cache/syscache.c.</I>
<I>src/backend/utils/cache/lsyscache.c</I> contains many
column-specific cache lookup functions.</P>
<P>The rows returned are cache-owned versions of the heap rows.
Therefore, you must not modify or delete the tuple returned by
<I>SearchSysCache()</I>. What you <I>should</I> do is release it
with <I>ReleaseSysCache()</I> when you are done using it; this
informs the cache that it can discard that tuple if necessary. If
you neglect to call <I>ReleaseSysCache()</I>, then the cache entry
will remain locked in the cache until end of transaction, which is
tolerable but not very desirable.</P>
<P>If you can't use the system cache, you will need to retrieve the
data directly from the heap table, using the buffer cache that is
shared by all backends. The backend automatically takes care of
loading the rows into the buffer cache.</P>
<P>Open the table with <I>heap_open().</I> You can then start a
table scan with <I>heap_beginscan(),</I> then use
<I>heap_getnext()</I> and continue as long as
<I>HeapTupleIsValid()</I> returns true. Then do a
<I>heap_endscan().</I> <I>Keys</I> can be assigned to the
<I>scan.</I> No indexes are used, so all rows are going to be
compared to the keys, and only the valid rows returned.</P>
<P>You can also use <I>heap_fetch()</I> to fetch rows by block
number/offset. While scans automatically lock/unlock rows from the
buffer cache, with <I>heap_fetch(),</I> you must pass a
<I>Buffer</I> pointer, and <I>ReleaseBuffer()</I> it when
completed.</P>
<P>Once you have the row, you can get data that is common to all
tuples, like <I>t_self</I> and <I>t_oid,</I> by merely accessing
the <I>HeapTuple</I> structure entries. If you need a
table-specific column, you should take the HeapTuple pointer, and
use the <I>GETSTRUCT()</I> macro to access the table-specific start
of the tuple. You then cast the pointer as a <I>Form_pg_proc</I>
pointer if you are accessing the pg_proc table, or
<I>Form_pg_type</I> if you are accessing pg_type. You can then
access the columns by using a structure pointer:</P>
<PRE>
<CODE>((Form_pg_class) GETSTRUCT(tuple))->relnatts
</CODE>
</PRE>
You must not directly change <I>live</I> tuples in this way. The
best way is to use <I>heap_modifytuple()</I> and pass it your
original tuple, and the values you want changed. It returns a
palloc'ed tuple, which you pass to <I>heap_replace().</I> You can
delete tuples by passing the tuple's <I>t_self</I> to
<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="2.2">2.2</A>) Why are table, column, type, function, view
names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
sometimes as <I>char *?</I></H3>
<P>Table, column, type, function, and view names are stored in system tables in columns of type <I>Name.</I> Name is a
fixed-length, null-terminated type of <I>NAMEDATALEN</I> bytes.
(The default value for NAMEDATALEN is 64 bytes.)</P>
<PRE>
<CODE>typedef struct nameData
{
char data[NAMEDATALEN];
} NameData;
typedef NameData *Name;
</CODE>
</PRE>
Table, column, type, function, and view names that come into the
backend via user queries are stored as variable-length,
null-terminated character strings.
<P>Many functions are called with both types of names, ie.
<I>heap_open().</I> Because the Name type is null-terminated, it is
safe to pass it to a function expecting a char *. Because there are
many cases where on-disk names(Name) are compared to user-supplied
names(char *), there are many cases where Name and char * are used
interchangeably.</P>
<H3><A name="2.3">2.3</A>) Why do we use <I>Node</I> and <I>List</I> to
make data structures?</H3>
<P>We do this because this allows a consistent way to pass data
inside the backend in a flexible way. Every node has a
<I>NodeTag</I> which specifies what type of data is inside the
Node. <I>Lists</I> are groups of <I>Nodes chained together as a
forward-linked list.</I></P>
<P>Here are some of the <I>List</I> manipulation commands:</P>
<BLOCKQUOTE>
<DL>
<DT>lfirst(i)</DT>
<DD>return the data at list element <I>i.</I></DD>
<DT>lnext(i)</DT>
<DD>return the next list element after <I>i.</I></DD>
<DT>foreach(i, list)</DT>
<DD>
loop through <I>list,</I> assigning each list element to
<I>i.</I> It is important to note that <I>i</I> is a List *,
not the data in the <I>List</I> element. You need to use
<I>lfirst(i)</I> to get at the data. Here is a typical code
snippet that loops through a List containing <I>Var *'s</I>
and processes each one:
<PRE>
<CODE>List *i, *list;
foreach(i, list)
{
Var *var = lfirst(i);
/* process var here */
}
</CODE>
</PRE>
</DD>
<DT>lcons(node, list)</DT>
<DD>add <I>node</I> to the front of <I>list,</I> or create a
new list with <I>node</I> if <I>list</I> is <I>NIL.</I></DD>
<DT>lappend(list, node)</DT>
<DD>add <I>node</I> to the end of <I>list.</I> This is more
expensive that lcons.</DD>
<DT>nconc(list1, list2)</DT>
<DD>Concat <I>list2</I> on to the end of <I>list1.</I></DD>
<DT>length(list)</DT>
<DD>return the length of the <I>list.</I></DD>
<DT>nth(i, list)</DT>
<DD>return the <I>i</I>'th element in <I>list.</I></DD>
<DT>lconsi, ...</DT>
<DD>There are integer versions of these: <I>lconsi, lappendi</I>,
etc. Also versions for OID lists: <I>lconso, lappendo</I>, etc.</DD>
</DL>
</BLOCKQUOTE>
You can print nodes easily inside <I>gdb.</I> First, to disable
output truncation when you use the gdb <I>print</I> command:
<PRE>
<CODE>(gdb) set print elements 0
</CODE>
</PRE>
Instead of printing values in gdb format, you can use the next two
commands to print out List, Node, and structure contents in a
verbose format that is easier to understand. List's are unrolled
into nodes, and nodes are printed in detail. The first prints in a
short format, and the second in a long format:
<PRE>
<CODE>(gdb) call print(any_pointer)
(gdb) call pprint(any_pointer)
</CODE>
</PRE>
The output appears in the postmaster log file, or on your screen if
you are running a backend directly without a postmaster.
<H3><A name="2.4">2.4</A>) I just added a field to a structure. What
else should I do?</H3>
<P>The structures passing around from the parser, rewrite,
optimizer, and executor require quite a bit of support. Most
structures have support routines in <I>src/backend/nodes</I> used
to create, copy, read, and output those structures. Make sure you
add support for your new field to these files. Find any other
places the structure may need code for your new field. <I>mkid</I>
is helpful with this (see above).</P>
<H3><A name="2.5">2.5</A>) Why do we use <I>palloc</I>() and
<I>pfree</I>() to allocate memory?</H3>
<P><I>palloc()</I> and <I>pfree()</I> are used in place of malloc()
and free() because we find it easier to automatically free all
memory allocated when a query completes. This assures us that all
memory that was allocated gets freed even if we have lost track of
where we allocated it. There are special non-query contexts that
memory can be allocated in. These affect when the allocated memory
is freed by the backend.</P>
<H3><A name="2.6">2.6</A>) What is ereport()?</H3>
<P><I>ereport()</I> is used to send messages to the front-end, and
optionally terminate the current query being processed. The first
parameter is an ereport level of <I>DEBUG</I> (levels 1-5), <I>LOG,</I>
<I>INFO,</I> <I>NOTICE,</I> <I>ERROR,</I> <I>FATAL,</I> or <I>PANIC.</I> <I>NOTICE</I> prints on the user's terminal and the
postmaster logs. <I>INFO</I> prints only to the user's terminal and
<I>LOG</I> prints only to the server logs. (These can be changed
from <I>postgresql.conf.</I>) <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>ereport</I> are a <I>printf</I>-style set of
parameters to print.</P>
<P><I>ereport(ERROR)</I> frees most memory and open file descriptors so
you don't need to clean these up before the call.</P>
<H3><A name="2.7">2.7</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>
<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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