Manual:Externallinks table

The externallinks table is where MediaWiki stores and tracks external links. This table was introduced after 1.5.

"DESCRIBE externallinks" in version 1.6 gives the following: +--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+
 * Field   | Type            | Null | Key | Default | Extra |
 * el_from | int(8) unsigned |      | MUL | 0       |       |
 * el_to   | blob            |      | MUL |         |       |
 * el_index | blob           |      | MUL |         |       |

"DESCRIBE externallinks" in version 1.8 gives the following: +--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+
 * Field   | Type            | Null | Key | Default | Extra |
 * el_from | int(8) unsigned | NO   | MUL | 0       |       |
 * el_to   | blob            | NO   | MUL | NULL    |       |
 * el_index | blob           | NO   | MUL | NULL    |       |

mysql> describe mw_externallinks; +--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+
 * Field   | Type            | Null | Key | Default | Extra |
 * el_from | int(8) unsigned | NO   | MUL | 0       |       |
 * el_to   | blob            | NO   | MUL | NULL    |       |
 * el_index | blob           | NO   | MUL | NULL    |       |

The el_from is the page identifier of the referring wiki page. el_to is the actual URL itself and is passed to the browser, whilst el_index is the same URL stripped of user and password information, then reversed for faster searching.

That is, a URL stored in el_to of 'http://user:password@sub.example.com/page.html' becomes 'http://com.example.sub./page.html', which allows searches of the form 'Show all links pointing to *.example.com'.