Added postmeta index for meta_key and meta_value to improve lookup query performance.

[sun]

Task

• Product: Add index meta_key_value

Description

• Lookup queries on specific meta fields in a very large database can be slow.
• core-standards already adds the meta_value index, but it only covers some strange cases in core and contributed plugins that are filtering by meta_value column only (e.g. to retrieve all distinct values) or the value is so unique among all meta fields that it doesn't require further filtering by key; e.g., a URL.
• The much more common case are lookup queries checking a particular meta_key for a particular meta_value.
• MySQL/MariaDB still uses the meta_value index for those, but EXPLAIN states that it still has to go through thousands of rows to reduce them to the given meta key, which is slow.

Examples

TODO: Need to find the actual queries that triggered this – for now here's a trivial:

$ wp db query "EXPLAIN SELECT post_id FROM wp_postmeta WHERE meta_key = '_pub_id' AND meta_value = '12345'"
+------+-------------+-------------+------+---------------------+------------+---------+-------+------+-------------+
| id   | select_type | table       | type | possible_keys       | key        | key_len | ref   | rows | Extra       |
+------+-------------+-------------+------+---------------------+------------+---------+-------+------+-------------+
|    1 | SIMPLE      | wp_postmeta | ref  | meta_key,meta_value | meta_value | 243     | const | 1    | Using where |
+------+-------------+-------------+------+---------------------+------------+---------+-------+------+-------------+

("using where" should ideally say "using index"; the real cases had 4k rows, the new index reduced them to <100)

We can see that MariaDB 10.4+ already has a built-in optimization, which doesn't need the new index, because it's using a second index to filter the first:

$ wp db query "EXPLAIN SELECT post_id FROM wp_postmeta WHERE meta_key = '_pub_id' AND meta_value = '12345'"
+------+-------------+-------------+------------+------------------------------------+---------------------+---------+-------+--------+---------------------------------+
| id   | select_type | table       | type       | possible_keys                      | key                 | key_len | ref   | rows   | Extra                           |
+------+-------------+-------------+------------+------------------------------------+---------------------+---------+-------+--------+---------------------------------+
|    1 | SIMPLE      | wp_postmeta | ref|filter | meta_key,meta_value,meta_key_value | meta_key|meta_value | 767|243 | const | 1 (0%) | Using where; Using rowid filter |
+------+-------------+-------------+------------+------------------------------------+---------------------+---------+-------+--------+---------------------------------+