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Commit9688c4e

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Make reduce_outer_joins() smarter about semijoins.
reduce_outer_joins() mistakenly treated a semijoin like a left join forpurposes of deciding whether not-null constraints created by the join'squals could be passed down into the join's left-hand side (possiblyresulting in outer-join simplification there). Actually, semijoin workslike inner join for this purpose, ie, we do not need to see any rows thatcan't possibly satisfy the quals. Hence, two-line fix to treat semi andinner joins alike. Per observation by Andres Freund about a performancegripe from Yazan Suleiman.Back-patch to 8.4, since this oversight has been there since the currenthandling of semijoins was implemented.
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‎src/backend/optimizer/prep/prepjointree.c

Lines changed: 8 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1840,6 +1840,11 @@ reduce_outer_joins_pass2(Node *jtnode,
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* is that we pass either the local or the upper constraints,
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* never both, to the children of an outer join.
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*
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* Note that a SEMI join works like an inner join here: it's okay
1844+
* to pass down both local and upper constraints. (There can't
1845+
* be any upper constraints affecting its inner side, but it's
1846+
* not worth having a separate code path to avoid passing them.)
1847+
*
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* At a FULL join we just punt and pass nothing down --- is it
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* possible to be smarter?
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*/
@@ -1849,7 +1854,7 @@ reduce_outer_joins_pass2(Node *jtnode,
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if (!computed_local_nonnullable_vars)
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local_nonnullable_vars=find_nonnullable_vars(j->quals);
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local_forced_null_vars=find_forced_null_vars(j->quals);
1852-
if (jointype==JOIN_INNER)
1857+
if (jointype==JOIN_INNER||jointype==JOIN_SEMI)
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{
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/* OK to merge upper and local constraints */
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local_nonnullable_rels=bms_add_members(local_nonnullable_rels,
@@ -1869,14 +1874,14 @@ reduce_outer_joins_pass2(Node *jtnode,
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if (left_state->contains_outer)
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{
1872-
if (jointype==JOIN_INNER)
1877+
if (jointype==JOIN_INNER||jointype==JOIN_SEMI)
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{
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/* pass union of local and upper constraints */
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pass_nonnullable_rels=local_nonnullable_rels;
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pass_nonnullable_vars=local_nonnullable_vars;
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pass_forced_null_vars=local_forced_null_vars;
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}
1879-
elseif (jointype!=JOIN_FULL)/* ie, LEFT/SEMI/ANTI */
1884+
elseif (jointype!=JOIN_FULL)/* ie, LEFT orANTI */
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{
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/* can't pass local constraints to non-nullable side */
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pass_nonnullable_rels=nonnullable_rels;

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