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MDEV-21383: Possible range plan is not used under certain conditions
[Variant 2 of the fix: collect the attached conditions] Problem: make_join_select() has a section of code which starts with "We plan to scan all rows. Check again if we should use an index." the code in that section will [unnecessarily] re-run the range optimizer using this condition: condition_attached_to_current_table AND current_table's_ON_expr Note that the original invocation of range optimizer in make_join_statistics was done using the whole select's WHERE condition. Taking the whole select's WHERE condition and using multiple-equalities allowed the range optimizer to infer more range restrictions. The fix: - Do range optimization using a condition that is an AND of this table's condition and all of the previous tables' conditions. - Also, fix the range optimizer to prefer SEL_ARGs with type=KEY_RANGE over SEL_ARGS with type=MAYBE_KEY, regardless of the key part. Computing key_and( SEL_ARG(type=MAYBE_KEY key_part=1), SEL_ARG(type=KEY_RANGE, key_part=2) ) will now produce the SEL_ARG with type=KEY_RANGE.
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