MDEV-38072 Optimizer choosing the wrong plan#4928
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When multiple indexes produce ordering that matches the ORDER BY ... LIMIT clause, like INDEX idx1(kp1, kp2) INDEX idx2(kp2) and the query with WHERE kp1=const ORDER BY kp2 LIMIT 2 then test_if_cheaper_ordering() in pre-11.0 versions will choose the index with the smallest KEY::user_defined_key_parts (idx2 in this example). This can produce a much worse query plan. The fix is to do what MariaDB 11.0 would do: use the index for which we have the cheapest access method. The cost of access method here takes into account that we will stop after producing #LIMIT rows. The fix is controlled by the @@optimizer_adjust_secondary_key_costs=fix_order_by_index_choice flag and is OFF by default.
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When multiple indexes produce ordering that matches the ORDER BY ... LIMIT clause, like
INDEX idx1(kp1, kp2)
INDEX idx2(kp2)
and the query with
WHERE kp1=const ORDER BY kp2 LIMIT 2
then test_if_cheaper_ordering() in pre-11.0 versions will choose the index with the smallest KEY::user_defined_key_parts (idx2 in this example). This can produce a much worse query plan.
The fix is to do what MariaDB 11.0 would do: use the index for which we have the cheapest access method. The cost of access method here takes into account that we will stop after producing #LIMIT rows.
The fix is controlled by the
@@optimizer_adjust_secondary_key_costs=fix_order_by_index_choice
flag and is OFF by default.