Manual:Searchindex table

The searchindex table is used to provide full text. Before MySQL 5.6, full text indexes were only provided by the MyISAM table engine, but not by InnoDB. The text table (cur table in 1.4 and earlier) however uses the InnoDB type to improve concurrency, so a copy of the current page text was required in order to make use of these indexes. This is the reason why this table is there. If using Postgres, this table does not exist: the full text information is stored as columns in the and  tables directly.

si_page
Key to .

si_title
Munged version of page title.

si_text
Munged version of the current text of the page.

Details
Indexes for this, using en wikipedia for cardinality:  +-++--+--+-+---+-+--++--++-+ +-++--+--+-+---+-+--++--++-+ +-++--+--+-+---+-+--++--++-+
 * Table      | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
 * searchindex |         0 | si_page  |            1 | si_page     | A         |      797275 |     NULL | NULL   |      | BTREE      |         |
 * searchindex |         1 | si_title |            1 | si_title    | A         |      265758 |     NULL | NULL   |      | FULLTEXT   |         |

A common search query is of the form:

In MediaWiki version 1.4 and earlier:

Explain shows the following: +-++---+---+-+++-+ +-++---+---+-+++-+ +-++---+---+-+++-+
 * table      | type   | possible_keys                             | key           | key_len | ref        | rows   | Extra       |
 * cur        | range  | cur_id,cur_namespace,name_title_timestamp | cur_namespace |       1 | NULL       | 317499 | Using where |
 * searchindex | eq_ref | si_page                                  | si_page       |       4 | cur.cur_id |      1 | Using where |

Known problems
A range scan with namespaces containing a large number of records requires that a large portion of the searchindex table be loaded into RAM. This is very costly, in part because the searchindex table contains the full text of the article. A long term plan is to remove the text from the searchindex table. Temporary measures may involve adding fields to searchindex and indexing them (and using a self join to allow both that index and the fulltext index to be used) or adding a searchindex index to decrease the number of complete searchindex records which must be read to match the query.

Schema summary
+--+--+--+-+-+---+ +--+--+--+-+-+---+ +--+--+--+-+-+---+
 * Field   | Type             | Null | Key | Default | Extra |
 * si_page | int(10) unsigned | NO   | PRI | NULL    |       |
 * si_title | varchar(255)    | NO   | MUL |         |       |
 * si_text | mediumtext       | NO   | MUL | NULL    |       |

+--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+
 * Field   | Type            | Null | Key | Default | Extra |
 * si_page | int(8) unsigned | NO   | PRI | NULL    |       |
 * si_title | varchar(255)   | NO   | MUL | NULL    |       |
 * si_text | mediumtext      | NO   | MUL | NULL    |       |