WL#1222: We need to unpack packed MyISAM key blocks once per block when reading keys via my_rprev
Affects: Server-7.1 — Status: Un-Assigned — Priority: Medium
We need to extend mi_rprev to unpack key block once on load. Currently we call _mi_get_last_key for each key read, and if keys are packed, _mi_get_last_key is expensive. Implementing this task will speed up index scan in reverse order by 6-15 times.
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