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436 lines
20 KiB
Plaintext
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Developer's Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Fri Jun 9 21:54:54 EDT 2000
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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The most recent version of this document can be viewed at the
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postgreSQL Web site, http://PostgreSQL.org.
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_________________________________________________________________
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Questions
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1) What tools are available for developers?
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2) What books are good for developers?
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3) Why do we use palloc() and pfree() to allocate memory?
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4) Why do we use Node and List to make data structures?
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5) How do I add a feature or fix a bug?
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6) How do I download/update the current source tree?
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7) How do I test my changes?
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7) I just added a field to a structure. What else should I do?
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8) Why are table, column, type, function, view names sometimes
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referenced as Name or NameData, and sometimes as char *?
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9) How do I efficiently access information in tables from the backend
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code?
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10) What is elog()?
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11) What is configure all about?
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12) How do I add a new port?
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13) What is CommandCounterIncrement()?
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14) Why don't we use threads in the backend?
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_________________________________________________________________
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1) What tools are available for developers?
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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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/tools directory are designed for developers.
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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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Let me note some of these. If you point your browser at the
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file:/usr/local/src/pgsql/src/tools/backend/index.html directory, you
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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
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area. You can click on any flowchart box to see a description. If you
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then click on the directory name, you will be taken to the source
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directory, to browse the actual source code behind it. We also have
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several README files in some source directories to describe the
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function of the module. The browser will display these when you enter
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the directory also. The tools/backend directory is also contained on
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our web page under the title How PostgreSQL Processes a Query.
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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
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back out twice to return to the original function. Most editors
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support this via tags or etags files.
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Third, you need to get id-utils from:
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ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz
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ftp://tug.org/gnu/id-utils-3.2d.tar.gz
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ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz
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By running tools/make_mkid, an archive of source symbols can be
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created that can be rapidly queried like grep or edited. Others prefer
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glimpse.
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make_diff has tools to create patch diff files that can be applied to
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the distribution.
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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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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 & indenting 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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pgindent will the format code by specifying flags to your operating
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system's utility indent.
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pgindent is run on all source files just before each beta test period.
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It auto-formats all source files to make them consistent. Comment
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blocks that need specific line breaks should be formatted as block
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comments, where the comment starts as /*------. These comments will
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not be reformatted in any way. pginclude contains scripts used to add
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needed #include's to include files, and removed unneeded #include's.
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When adding system types, you will need to assign oids to them. There
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is also a script called unused_oids in pgsql/src/include/catalog that
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shows the unused oids.
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2) What books are good for developers?
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I have four good books, An Introduction to Database Systems, by C.J.
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Date, Addison, Wesley, A Guide to the SQL Standard, by C.J. Date, et.
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al, Addison, Wesley, Fundamentals of Database Systems, by Elmasri and
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Navathe, and Transaction Processing, by Jim Gray, Morgan, Kaufmann
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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 http://www.benchmarkresources.com.
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3) Why do we use palloc() and pfree() to allocate memory?
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palloc() and pfree() are used in place of malloc() and free() because
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we automatically free all memory allocated when a transaction
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completes. This makes it easier to make sure we free memory that gets
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allocated in one place, but only freed much later. There are several
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contexts that memory can be allocated in, and this controls when the
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allocated memory is automatically freed by the backend.
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4) Why do we use Node and List to make data structures?
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We do this because this allows a consistent way to pass data inside
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the backend in a flexible way. Every node has a NodeTag which
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specifies what type of data is inside the Node. Lists are groups of
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Nodes chained together as a forward-linked list.
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Here are some of the List manipulation commands:
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lfirst(i)
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return the data at list element i.
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lnext(i)
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return the next list element after i.
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foreach(i, list)
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loop through list, assigning each list element to i. It is
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important to note that i is a List *, not the data in the List
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element. You need to use lfirst(i) to get at the data. Here is
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a typical code snipped that loops through a List containing Var
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*'s and processes each one:
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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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lcons(node, list)
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add node to the front of list, or create a new list with node
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if list is NIL.
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lappend(list, node)
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add node to the end of list. This is more expensive that lcons.
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nconc(list1, list2)
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Concat list2 on to the end of list1.
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length(list)
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return the length of the list.
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nth(i, list)
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return the i'th element in list.
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lconsi, ...
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There are integer versions of these: lconsi, lappendi, nthi.
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List's 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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You can print nodes easily inside gdb. First, to disable output
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truncation when you use the gdb print command:
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(gdb) set print elements 0
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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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(gdb) call print(any_pointer)
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(gdb) call pprint(any_pointer)
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The output appears in the postmaster log file, or on your screen if
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you are running a backend directly without a postmaster.
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5) How do I add a feature or fix a bug?
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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.
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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,
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then looking at other areas in the code where similar things are done,
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and by the time I am finished, the patch is quite small and compact.
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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.
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6) How do I download/update the current source tree?
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There are several ways to obtain the source tree. Occasional
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developers 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
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have 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.
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To update the source tree, there are two ways. You can generate a
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patch against your current source tree, perhaps using the make_diff
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tools 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
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we are in beta testing, the developers may wait for the final release
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before applying your patches.
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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.
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6) How do I test my changes?
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First, use psql to make sure it is working as you expect. Then run
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src/test/regress and get the output of src/test/regress/checkresults
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with and without your changes, to see that your patch does not change
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the regression test in unexpected ways. This practice has saved me
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many times. The regression tests test the code in ways I would never
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do, and has caught many bugs in my patches. By finding the problems
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now, you save yourself a lot of debugging later when things are
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broken, and you can't figure out when it happened.
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7) I just added a field to a structure. What else should I do?
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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 src/backend/nodes used to create, copy, read, and output
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those structures. Make sure you add support for your new field to
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these files. Find any other places the structure may need code for
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your new field. mkid is helpful with this (see above).
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8) Why are table, column, type, function, view names sometimes referenced as
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Name or NameData, and sometimes as char *?
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Table, column, type, function, and view names are stored in system
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tables in columns of type Name. Name is a fixed-length,
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null-terminated type of NAMEDATALEN bytes. (The default value for
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NAMEDATALEN is 32 bytes.)
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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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Table, column, type, function, and view names that come into the
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backend via user queries are stored as variable-length,
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null-terminated character strings.
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Many functions are called with both types of names, ie. heap_open().
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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
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on-disk names(Name) are compared to user-supplied names(char *), there
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are many cases where Name and char * are used interchangeably.
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9) How do I efficiently access information in tables from the backend code?
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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, SearchSysCache() and related functions allow you
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to query the system catalogs. This is the preferred way to access
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system tables, because the first call to the cache loads the needed
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rows, and future requests can return the results without accessing the
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base table. The caches use system table indexes to look up tuples. A
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list of available caches is located in
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src/backend/utils/cache/syscache.c.
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src/backend/utils/cache/lsyscache.c contains many column-specific
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cache lookup functions.
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The rows returned are cache-owned versions of the heap rows.
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Therefore, you must not modify or delete the tuple returned by
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SearchSysCache(). What you should do is release it with
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ReleaseSysCache() when you are done using it; this informs the cache
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that it can discard that tuple if necessary. If you neglect to call
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ReleaseSysCache(), then the cache entry will remain locked in the
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cache until end of transaction, which is tolerable but not very
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desirable.
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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.
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Open the table with heap_open(). You can then start a table scan with
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heap_beginscan(), then use heap_getnext() and continue as long as
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HeapTupleIsValid() returns true. Then do a heap_endscan(). Keys can be
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assigned to the scan. No indexes are used, so all rows are going to be
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compared to the keys, and only the valid rows returned.
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You can also use heap_fetch() to fetch rows by block number/offset.
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While scans automatically lock/unlock rows from the buffer cache, with
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heap_fetch(), you must pass a Buffer pointer, and ReleaseBuffer() it
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when completed.
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Once you have the row, you can get data that is common to all tuples,
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like t_self and t_oid, by merely accessing the HeapTuple structure
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entries. If you need a table-specific column, you should take the
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HeapTuple pointer, and use the GETSTRUCT() 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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Form_pg_proc pointer if you are accessing the pg_proc table, or
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Form_pg_type if you are accessing pg_type. You can then access the
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columns by using a structure pointer:
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((Form_pg_class) GETSTRUCT(tuple))->relnatts
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You must not directly change live tuples in this way. The best way is
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to use heap_modifytuple() and pass it your original tuple, and the
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values you want changed. It returns a palloc'ed tuple, which you pass
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to heap_replace(). You can delete tuples by passing the tuple's t_self
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to heap_destroy(). You use t_self for heap_update() too. Remember,
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tuples can be either system cache copies, which may go away after you
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call ReleaseSysCache(), or read directly from disk buffers, which go
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away when you heap_getnext(), heap_endscan, or ReleaseBuffer(), in the
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heap_fetch() case. Or it may be a palloc'ed tuple, that you must
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pfree() when finished.
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10) What is elog()?
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elog() is used to send messages to the front-end, and optionally
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terminate the current query being processed. The first parameter is an
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elog level of NOTICE, DEBUG, ERROR, or FATAL. NOTICE prints on the
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user's terminal and the postmaster logs. DEBUG prints only in the
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postmaster logs. ERROR prints in both places, and terminates the
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current query, never returning from the call. FATAL terminates the
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backend process. The remaining parameters of elog are a printf-style
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set of parameters to print.
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11) What is configure all about?
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The files configure and configure.in are part of the GNU autoconf
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package. Configure allows us to test for various capabilities of the
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OS, and to set variables that can then be tested in C programs and
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Makefiles. Autoconf is installed on the PostgreSQL main server. To add
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options to configure, edit configure.in, and then run autoconf to
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generate configure.
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When configure is run by the user, it tests various OS capabilities,
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stores those in config.status and config.cache, and modifies a list of
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*.in files. For example, if there exists a Makefile.in, configure
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generates a Makefile that contains substitutions for all @var@
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parameters found by configure.
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When you need to edit files, make sure you don't waste time modifying
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files generated by configure. Edit the *.in file, and re-run configure
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to recreate the needed file. If you run make distclean from the
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top-level source directory, all files derived by configure are
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removed, so you see only the file contained in the source
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distribution.
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12) How do I add a new port?
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There are a variety of places that need to be modified to add a new
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port. First, start in the src/template directory. Add an appropriate
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entry for your OS. Also, use src/config.guess to add your OS to
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src/template/.similar. You shouldn't match the OS version exactly. The
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configure test will look for an exact OS version number, and if not
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found, find a match without version number. Edit src/configure.in to
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add your new OS. (See configure item above.) You will need to run
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autoconf, or patch src/configure too.
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Then, check src/include/port and add your new OS file, with
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appropriate values. Hopefully, there is already locking code in
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src/include/storage/s_lock.h for your CPU. There is also a
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src/makefiles directory for port-specific Makefile handling. There is
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a backend/port directory if you need special files for your OS.
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13) What is CommandCounterIncrement()?
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Normally, transactions can not see the rows they modify. This allows
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UPDATE foo SET x = x + 1 to work correctly.
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However, there are cases where a transactions needs to see rows
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affected in previous parts of the transaction. This is accomplished
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using a Command Counter. Incrementing the counter allows transactions
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to be broken into pieces so each piece can see rows modified by
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previous pieces. CommandCounterIncrement() increments the Command
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Counter, creating a new part of the transaction.
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14) Why don't we use threads in the backend?
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There are several reasons threads are not used:
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* Historically, threads were unsupported and buggy.
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* An error in one backend can corrupt other backends.
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* Speed improvements using threads are small compared to the
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remaining backend startup time.
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* The backend code would be more complex.
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