MDEV-28817: Derived table elimination fails when field aliases are used#4866
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MDEV-28817: Derived table elimination fails when field aliases are used#4866
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When a derived table's SELECT list contains the same column under multiple aliases (e.g. "SELECT a, b, a as a2 FROM t2 GROUP BY a, b"), the table elimination logic failed to recognize that the aliased column is also part of the GROUP BY pseudo-key. The root cause was in find_field_in_list() which only recorded the first matching position in the SELECT list for each GROUP BY element. If the ON clause referenced the column via a different alias (a later position), covers_field() would not find it in the pseudo-key bitmap. Fix: find_field_in_list() now marks ALL matching positions in the SELECT list, so every alias of a GROUP BY column is recognized as part of the pseudo-key.
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When a derived table's SELECT list contains the same column under multiple aliases (e.g. "SELECT a, b, a as a2 FROM t2 GROUP BY a, b"), the table elimination logic failed to recognize that the aliased column is also part of the GROUP BY pseudo-key.
The root cause was in find_field_in_list() which only recorded the first matching position in the SELECT list for each GROUP BY element. If the ON clause referenced the column via a different alias (a later position), covers_field() would not find it in the pseudo-key bitmap.
Fix: find_field_in_list() now marks ALL matching positions in the SELECT list, so every alias of a GROUP BY column is recognized as part of the pseudo-key.