Sergei Petrunia 4e8c17534d MDEV-38072 Optimizer choosing the wrong plan ...

(Variant 3, more comments)
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.

2026-04-16 09:26:39 +10:00

18 KiB

Raw Permalink Blame History

View Raw