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diff --git a/doc/src/FAQ/FAQ_DEV.html b/doc/src/FAQ/FAQ_DEV.html index 9775e6c262b..09d02f6a73c 100644 --- a/doc/src/FAQ/FAQ_DEV.html +++ b/doc/src/FAQ/FAQ_DEV.html @@ -2,866 +2,13 @@ <HTML> <HEAD> - <META name="generator" content="HTML Tidy, see www.w3.org"> - <TITLE>PostgreSQL Developers FAQ</TITLE> </HEAD> <BODY bgcolor="#FFFFFF" text="#000000" link="#FF0000" vlink="#A00000" alink="#0000FF"> - <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> + <P>The developer FAQ can be found on the PostgreSQL wiki:</P> +<P><A href="http://wiki.postgresql.org/wiki/Development_information">http://wiki.postgresql.org/wiki/Development_information</A></P> </BODY> </HTML> |