This document describes the state of Postgres support in MediaWiki.


== Overview ==

Support for PostgreSQL has been available since version 1.7
of MediaWiki, and is fairly well maintained. The main code
is very well integrated, while extensions are very hit and miss.
Still, it is probably the most supported database after MySQL.
Much of the work in making MediaWiki database-agnostic came
about through the work of creating Postgres support.


== Required versions ==

The current minimum version of PostgreSQL for MediaWiki is 8.1.
It is expected that this will be raised to 8.3 at some point,
as 8.1 and 8.2 are nearing end of life.



== Database schema ==

Postgres has its own schema file at maintenance/postgres/tables.sql.

The goal is to keep this file as close as possible to the canonical
schema at maintenance/tables.sql, but without copying over
all the usage comments. General notes on the conversion:

* The use of a true TIMESTAMP rather than the text string that
MySQL uses is highly encouraged. There are still places in the
code (especially extensions) which make assumptions about the
textual nature of timestamp fields, but these can almost always
be programmed around.

* Although Postgres has a true BOOLEAN type, boolean columns
are always mapped to SMALLINT, as the code does not always treat
the column as a boolean (which is limited to accepting true,
false, 0, 1, t, or f)

* The default data type for all VARCHAR, CHAR, and VARBINARY
columns should simply be TEXT. The only exception is
when VARBINARY is used to store true binary data, such as
the math_inputhash column, in which case BYTEA should be used.

* All integer variants should generally be mapped to INTEGER.
There is small-to-no advantage in using SMALLINT versus
INTEGER in Postgres, and the possibility of running out of
room outweighs such concerns. The columns that are BIGINT
in other schemas should be INTEGER as well, as none of them
so far are even remotely likely to reach the 32 billion
limit of an INTEGER.

* Blobs (blob, tinyblog, mediumblob) should be mapped to TEXT
whenever possible, and to BYTEA if they are known to contain
binary data.

* All length modifiers on data types should be removed. If
they are on an INTEGER, it's probably an error, and if on
any text-based field, simply using TEXT is preferred.

* Sequences should be explicitly named rather than using
SERIAL, as the code can depend on having a specific name.

* Foreign keys should be used when possible. This makes things
both easier and harder in the code, but most of the major
problems have now been overcome. Always add an explicit ON DELETE
clause, and consider carefully what choice to use (all things
considered, prefer CASCADE).

* The use of CIDR should be done very carefully, because the code
will sometimes want to store things such as an empty string or
other non-IP value in the column. When in doubt, use TEXT.

* Indexes should be created using the original MySQL tables.sql
as a guide, but keeping in mind the ability of Postgres to use
partial indexes, functional indexes, and bitmaps. The index names
should be logical but are not too important, as they are never
referenced directly by the code (unlike sequence names). Most of
the indexes in the file as of this writing are there due to production
testing of expensive queries on a busy wiki.


== Keeping in sync with tables.sql ==

The script maintenance/postgres/compare_schemas.pl should be
periodically run. It will parse both "tables.sql" files and
produce any differences found. Such differences should be fixed
or exceptions specifically carved out by editing the script
itself. This script has also been very useful in finding problems
in maintenance/tables.sql itself, as it is very strict in the
format it expects things to be in. :)


== MySQL differences ==

The major differences between MySQL and Postgres are represented as 
methods in the Database class. For example, implicitGroupby() is 
true for MySQL and false for Postgres. This means that in those 
places where the code does not add all the non-aggregate items 
from the SELECT clause to the GROUP BY, we can add them in, but in 
a conditional manner with the above method, as simply adding them 
all in to the main query may cause performance problems with 
MySQL.


== Getting help ==

In addition to the normal venues (MediaWiki mailing lists
and IRC channels), the #postgresql channel on irc.freenode.net
is a friendly and expert resource if you should encounter a
problem with your Postgres-enabled MediaWiki.
