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https://issues.apache.org/jira/browse/CALCITE-3760?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17376160#comment-17376160
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Shuo Cheng commented on CALCITE-3760:
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> As we discussed recently, it would be illegal to merge those Projects because
> of the UDF.
[~julianhyde] This is also the problem we encountered, merging two Projects
with non-deterministic operator may leading to the operator being executed
multiple times, thus generating different results, which is not expected. I'm
wondering is there any consensus being achieved on the discussion? (i.e., rules
and rewriting of RexNode should deal with non-deterministic operators
carefully...)
> Rewriting non-deterministic function can break query semantics
> --------------------------------------------------------------
>
> Key: CALCITE-3760
> URL: https://issues.apache.org/jira/browse/CALCITE-3760
> Project: Calcite
> Issue Type: Bug
> Components: core
> Reporter: Jin Xing
> Assignee: Jin Xing
> Priority: Major
> Labels: pull-request-available
> Time Spent: 10m
> Remaining Estimate: 0h
>
> Calcite rewrite some *SqlFunctions* during validation. But whether the
> function is deterministic is not considered. For a non-deterministic
> operator, the rewriting can break semantics. Additionally there's no
> interface for user to specify the determinism for a UDF/UDAF.
> Say I have non-deterministic UDF & UDAF and run sql like below
> {code:java}
> select coalesce(udf(col0), 100) from foo;
> select nullif(udaf(col0), 1024) from foo;{code}
> They will be rewritten as
> {code:java}
> select case when udf(col0) is not null then udf(col0) else 100 end
> from foo;
> select case when udaf(col0)=1024 then null udaf(col0)
> from foo{code}
> As we can see that non-deterministic UDF & UDAF are called multiple times
> after written. Thus the condition in WHEN clause might NOT be held all the
> time.
> We need to provide an interface for user to specify the determinism in
> UDF/UDAF and consider whether a SqlNode is deterministic when rewriting.
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