Manual:Externallinks table

The externallinks table is where MediaWiki stores and tracks external links.

el_id
The primary key. Presently not used for anything, but will help with online schema changes.

el_from
The page id of the referring wiki page.

el_to
The actual URL itself. It is passed to the browser.

el_index
This is the same URL as  search-optimized: username and password information is stripped, and the other components are reversed for faster searching, so  becomes , which allows searches of the form 'Show all links pointing to *.example.com'.

el_index_60
This is el_index truncated to 60 bytes to allow for sortable queries that aren't supported by a partial index.

el_to_domain_index
This is similar to el_index but only includes the protocol specification and domain

el_to_path
Only the path of el_to

Schema summary
++--+--+-+-++ ++--+--+-+-++ ++--+--+-+-++
 * Field             | Type             | Null | Key | Default | Extra          |
 * el_id             | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
 * el_from           | int(10) unsigned | NO   | MUL | 0       |                |
 * el_to             | blob             | NO   | MUL | NULL    |                |
 * el_index          | blob             | NO   | MUL | NULL    |                |
 * el_index_60       | varbinary(60)    | NO   | MUL | NULL    |                |
 * el_to_domain_index | varbinary(255)  | NO   | MUL |         |                |
 * el_to_path        | blob             | YES  |     | NULL    |                |

+-+--+--+-+-++ +-+--+--+-+-++ +-+--+--+-+-++
 * Field      | Type             | Null | Key | Default | Extra          |
 * el_id      | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
 * el_from    | int(10) unsigned | NO   | MUL | 0       |                |
 * el_to      | blob             | NO   | MUL | NULL    |                |
 * el_index   | blob             | NO   | MUL | NULL    |                |
 * el_index_60 | varbinary(60)   | NO   | MUL | NULL    |                |

+-+--+--+-+-++ +-+--+--+-+-++ +-+--+--+-+-++
 * Field      | Type             | Null | Key | Default | Extra          |
 * el_id      | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
 * el_from    | int(10) unsigned | NO   | MUL | 0       |                |
 * el_to      | blob             | NO   | MUL | NULL    |                |
 * el_index   | blob             | NO   | MUL | NULL    |                |
 * el_index_60 | varbinary(60)   | NO   | MUL |         |                |

+--+--+--+-+-++ +--+--+--+-+-++ +--+--+--+-+-++
 * Field   | Type             | Null | Key | Default | Extra          |
 * el_id   | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
 * el_from | int(10) unsigned | NO   | MUL | 0       |                |
 * el_to   | blob             | NO   | MUL | NULL    |                |
 * el_index | blob            | NO   | MUL | NULL    |                |

+--+--+--+-+-+---+ +--+--+--+-+-+---+ +--+--+--+-+-+---+
 * Field   | Type             | Null | Key | Default | Extra |
 * el_from | int(10) unsigned | NO   | MUL | 0       |       |
 * el_to   | blob             | NO   | MUL | NULL    |       |
 * el_index | blob            | NO   | MUL | NULL    |       |

+--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+
 * 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    |       |

Indexes
 +---+++--++---+-+--++--++-+---+ +---+++--++---+-+--++--++-+---+ +---+++--++---+-+--++--++-+---+
 * Table        | Non_unique | Key_name                   | Seq_in_index | Column_name        | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
 * externallinks |         0 | PRIMARY                    |            1 | el_id              | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_from                    |            1 | el_from            | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_to                      |            1 | el_to              | A         |           0 |       60 | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_to                      |            2 | el_from            | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_index                   |            1 | el_index           | A         |           0 |       60 | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_index_60                |            1 | el_index_60        | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_index_60                |            2 | el_id              | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_from_index_60           |            1 | el_from            | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_from_index_60           |            2 | el_index_60        | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_from_index_60           |            3 | el_id              | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_to_domain_index_to_path |            1 | el_to_domain_index | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
 * externallinks |         1 | el_to_domain_index_to_path |            2 | el_to_path         | A         |           0 |       60 | NULL   | YES  | BTREE      |         |               |