Improve support for correlated subqueries with GROUP BY or LIMIT #8554
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kasiafi
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| } | ||
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| // based on `UnwrapCastInComparison.Visitor.hasInjectiveImplicitCoercion()` | ||
| private boolean hasInjectiveCoercion(Type source, Type target) |
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We may want to move this to TypeCoercion
| // they are derived from the filter predicate, e.g. | ||
| // a = corr --> a is constant | ||
| // b = f(corr_1, corr_2, ...) --> b is constant provided that f is deterministic | ||
| // cast(c AS ...) = corr --> c is constant provided that cast is injective |
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I wonder if this case could be handled by expanding the UnwrapCastInComparison optimizer to deal with correlation symbols on the right hand side. In that case, the expression would be turned to something like:
c = cast(corr as ...)
and the second rule would apply.
That way, we don't need the decorrelator to deal with cast and checking whether it's injective at all.
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The transformation which is done in UnwrapCastInComparison is based on knowing the actual value of the right-hand-side expression. Here, we only know that it's constant. If apply the cast to corr, it might turn out to be a truncating cast, and the result of the comparison might change. Or we could get a cast exception.
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Fair enough. In the future, we might want to add an optimization that works when the right side is constant -- it would be a weaker form of UnwrapCastInComparison.
kasiafi
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| @@ -128,6 +128,30 @@ public boolean isTypeOnlyCoercion(Type source, Type result) | |||
| return false; | |||
| } | |||
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| // based on `UnwrapCastInComparison.Visitor.hasInjectiveImplicitCoercion()` | |||
| public boolean isInjectiveCoercion(Type source, Type result) | |||
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This should be used by UnwrapCastInComparison, too
kasiafi
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This change introduces a new policy of determining which subquery symbols are constant. Before, only symbols equal to some correlation symbols were recognized as constant. After this change, also deterministic combinations of correlation symbols and coerced subquery symbols are recognized as constant. Recognizing more subquery symbols as constant helps decorrelate or improves plans for correlated queries with: GROUP BY, LIMIT, ORDER BY + LIMIT.
kasiafi
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This change introduces a new policy of determining which subquery
symbols are constant.
Before, only symbols equal to some correlation symbols were recognized
as constant.
After this change, also deterministic combinations of correlation
symbols and coerced subquery symbols are recognized as constant.
Recognizing more subquery symbols as constant helps decorrelate
or improves plans for correlated queries with: GROUP BY, LIMIT,
ORDER BY + LIMIT.