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4bf7c966b3
(With trivial fixes by sergey@mariadb.com) Added option fix_innodb_cardinality to optimizer_adjust_secondary_key_costs Using fix_innodb_cardinality disables the 'divide by 2' of rec_per_key_int in InnoDB that in effect doubles the Cardinality for secondary keys. This has the biggest effect for indexes where a few rows has the same key value. Using this may also cause table scans for very small tables (which in some cases may be better than an index scan). The user visible effect is that 'SHOW INDEX FROM table_name' will for InnoDB show the true Cardinality (and not 2x the real value). It will also allow the optimizer to chose a better index in some cases as the division by 2 could have a bad effect for tables with 2-5 identical values per key. A few notes about using fix_innodb_cardinality: - It has direct affect for SHOW INDEX FROM table_name. SHOW INDEX will also update the statistics in table share. - The effect of fix_innodb_cardinality for query plans or EXPLAIN is only visible after first open of the table. This is why one must do a flush tables or use SHOW INDEX for the option to take effect. - Using fix_innodb_cardinality can thus affect all user in their query plans if they are using the same tables. Because of this, it is strongly recommended that one uses optimizer_adjust_secondary_key_costs=fix_innodb_cardinality mainly in configuration files to not cause issues for other users. |
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