Details
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Bug
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Status: Closed (View Workflow)
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Major
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Resolution: Fixed
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10.5, 10.6, 10.11
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None
Description
InnoDB is known to produce overly-optimistic index estimates for index lookups.
This was done intentionally, see this piece in ha_innobase::info_low:
/* Since MySQL seems to favor table scans |
too much over index searches, we pretend
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index selectivity is 2 times better than
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our estimate: */
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rec_per_key_int = rec_per_key_int / 2;
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However in some cases that can cause poor query plans.
in 11.0, this was removed by this commit:
commit 01c82173dd8b6ea627dec949dbc57dfcc94a656d
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Author: Monty <monty@mariadb.org>
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Date: Thu Jan 26 11:38:31 2023 +0200
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Removed /2 of InnoDB ref_per_key[] estimates
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The original code was there to favor index search over table scan.
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This is not needed anymore as the cost calculations for table scans
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and index lookups are now more exact.
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This MDEV is about adding a capability to remove the /2 in versions before 11.0