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https://issues.apache.org/jira/browse/CALCITE-7812?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18121992#comment-18121992
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Lino Rosa commented on CALCITE-7812:
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Humm, it's one thing if Calcite's runtime hardcodes these expectations, such as
that SUM returns NOT NULL. But if Calcite's runtime is instead evaluating
SqlReturnTypeInference then do you still see any problems with my approach?
I guess that's what you're afraid of, that there is hardwired logic that
expects the existing type signatures to be what they are, and if we open the
pandora's box we may get all sorts of failures? But aren't these just bugs to
be fixed? Only those who override the built-in functions would be suffering the
consequences anyway.
What I'm proposing is technically a refactor that opens the door to what I want
to do, not necessarily any semantic logic. Basically allowing overriding the
SqlFunctions inside SqlStdOperatorTable. Currently that's not possible because
of two things (afaik):
1. These functions get instantiated from deep inside Calcite. When this happens
we would want to instantiate the overrides, not the original signatures.
2. There are comparisons by reference from deep inside Calcite. When this
happens we would want to compare against the overrides, not the original
signatures.
---
If you're still not convinced, do you have any other ideas around this issue?
Otherwise there are certain query shapes that could never be compiled to Spark,
and likely other dialects too.
> Implicit type coercion emits CASTs in unparsed SQL that the target dialect
> rejects
> ----------------------------------------------------------------------------------
>
> Key: CALCITE-7812
> URL: https://issues.apache.org/jira/browse/CALCITE-7812
> Project: Calcite
> Issue Type: Bug
> Reporter: Lino Rosa
> Assignee: krooswu
> Priority: Major
>
> h2. Problem
> When type coercion is on, {{TypeCoercionImpl}} rewrites the validated
> {{SqlNode}} tree. It wraps operands and select items in {{{}CAST(x AS T){}}},
> where {{T}} is the type Calcite's own rules chose. These casts become
> ordinary {{RexCall(CAST)}} nodes after {{{}SqlToRelConverter{}}}.
> {{RelToSqlConverter}} then unparses them into the SQL sent to the target
> engine.
> That is fine only when Calcite and the engine type the expression in a
> compatible way. Otherwise it generates invalid queries.
> h4. Example: struct fields and {{SUM}} nullability on Spark
> {code:java}
> -- target: t2 (s ROW(a DOUBLE))
> -- source: t1 (k INT NOT NULL, x BIGINT NOT NULL)
> INSERT INTO t2 (s)
> SELECT ROW(SUM(x)) FROM t1 GROUP BY k {code}
> # Calcite types {{SUM}} as {{{}BIGINT NOT NULL{}}}, because {{x}} is {{NOT
> NULL}} and the query is grouped.
> # {{coerceColumnType}} wraps the row in a {{CAST}} to the target struct type.
> # {{SqlTypeUtil.convertTypeToSpec}} loses per-field nullability
> (CALCITE-6932). The unparsed cast spells the field {{{}NOT NULL{}}}.
> # Spark types {{SUM}} as nullable. It refuses to cast a struct with a
> nullable field to one with a {{NOT NULL}} field, so the query fails at
> analysis. Fixing CALCITE-6932 alone would not settle this. The cast still
> carries Calcite's view of nullability, which is correct for Calcite and wrong
> for Spark. The engine would have accepted the uncast expression.
> h4. Existing workaround is not enough
> {{SqlDialect.supportsImplicitTypeCoercion}} together with
> {{SqlImplementor.stripCastFromString}} already recognizes that engines coerce
> for themselves. Its scope is limited but its existing is promising and may
> lead to a fix.
> h3. Proposals
> h4. 1. Carry the coerced type outside the tree
> h4. The validator would record the coerced type without rewriting the tree,
> and conversion would pick it up from there. This is the cleanest option but
> hard today, because the tree is the only place coercion is stored. Much of
> the downstream logic (e.g. {{FamilyOperandTypeChecker, }}{{SqlToRelConverter)
> rely on the existence of this CAST.}}
> h4. 2. Mark coercion casts, then let the dialect decide (preferred)
> h4. Keep inserting a cast, but make it distinguishable from a user cast. Two
> ways to do that:
> * A dedicated operator of kind {{{}CAST{}}}, for example
> {{{}SqlStdOperatorTable.IMPLICIT_CAST{}}}.
> * A flag on the cast.
> The marker has to survive {{{{{}SqlToRelConverter{}}}}} and appear on the
> {{{}RexCall{}}}. Because its kind stays {{{}CAST{}}}, {{RexSimplify}} and the
> rules keep treating it as a cast.
> And here we go back to {{SqlDialect.supportsImplicitTypeCoercion.}} We can
> now simply unparse this marker as either a \{{CAST }}or not unparse anything
> depending on that flag.
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