Manual:Querycache table

The querycache table is used for caching expensive grouped queries.

qc_type
A key name, generally the base name of the special page.

qc_value
Some sort of stored value. Sizes, counts...

qc_namespace
Target namespace; used in conjunction with gc_title.

qc_title
Target title; used in conjunction with gc_namespace.

Versions
mysql> describe querycache; +--+--+--+-+-+---+ +--+--+--+-+-+---+ +--+--+--+-+-+---+
 * Field       | Type             | Null | Key | Default | Extra |
 * qc_type     | varbinary(32)    | NO   | MUL | NULL    |       |
 * qc_value    | int(10) unsigned | NO   |     | 0       |       |
 * qc_namespace | int(11)         | NO   |     | 0       |       |
 * qc_title    | varbinary(255)   | NO   |     |         |       |

describe querycache; +--+--+--+-+-+---+ +--+--+--+-+-+---+ +--+--+--+-+-+---+
 * Field       | Type                 | Null | Key | Default | Extra |
 * qc_type     | varbinary(32)        |      | MUL |         |       |
 * qc_value    | int(10) unsigned     |      |     | 0       |       |
 * qc_namespace | tinyint(11) unsigned |     |     | 0       |       |
 * qc_title    | varchar(255)         |      |     |         |       |

describe querycache; in MediaWiki 1.8 gives the following:

+--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+
 * Field       | Type            | Null | Key | Default | Extra |
 * qc_type     | char(32)        | NO   | MUL | NULL    |       |
 * qc_value    | int(5) unsigned | NO   |     | 0       |       |
 * qc_namespace | int(11)        | NO   |     | 0       |       |
 * qc_title    | char(255)       | NO   |     | NULL    |       |

+--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+
 * Field       | Type            | Null | Key | Default | Extra |
 * qc_type     | char(32)        | NO   | MUL | NULL    |       |
 * qc_value    | int(5) unsigned | NO   |     | 0       |       |
 * qc_namespace | int(11)        | NO   |     | 0       |       |
 * qc_title    | char(255)       | NO   |     | NULL    |       |

+--+--+--+-+-+---+ +--+--+--+-+-+---+ +--+--+--+-+-+---+
 * Field       | Type             | Null | Key | Default | Extra |
 * qc_type     | varbinary(32)    | NO   | MUL | NULL    |       |
 * qc_value    | int(10) unsigned | NO   |     | 0       |       |
 * qc_namespace | int(11)         | NO   |     | 0       |       |
 * qc_title    | varchar(255)     | NO   |     | NULL    |       |

+--+-+--+-+-+---+ +--+-+--+-+-+---+ +--+-+--+-+-+---+
 * Field       | Type            | Null | Key | Default | Extra |
 * qc_type     | char(32)        | NO   | MUL | NULL    |       |
 * qc_value    | int(5) unsigned | NO   |     | 0       |       |
 * qc_namespace | int(11)        | NO   |     | 0       |       |
 * qc_title    | char(255)       | NO   |     | NULL    |       |