Paul Rogers created IMPALA-7737:
-----------------------------------
Summary: Extend CASE code generation to cache repeated
sub-expressions
Key: IMPALA-7737
URL: https://issues.apache.org/jira/browse/IMPALA-7737
Project: IMPALA
Issue Type: Improvement
Affects Versions: Impala 3.0
Reporter: Paul Rogers
IMPALA-7655 suggests rewriting several conditional functions which can result
in repeated evaluation of the same sub-expression. For example:
{code:sql}
ISNULL(expr, ifNull)
{code}
Might become:
{code:sql}
CASE WHEN expr IS NULL THEN ifNull ELSE expr END
{code}
In the original (interpreted) form, {{expr}} is evaluated once but used twice.
In the code generated form for {{CASE}}, {{expr}} will be evaluated twice,
removing some of the benefit of the rewrite.
This ticket requests to modify the FE and BE to handle common subexpressions.
In the most general case, the FE would search for such common subexpressions.
For our purposes, a more narrow approach is possible: the rewrite rule would
simply label such expressions during the rewrite.
Note that the label must survive subsequent rewrites (or such rewrites must be
done before the rewrite into CASE.) For example:
{code:sql}
ISNULL(1 + 2 + 3 + col, 5)
{code}
Will be rewritten to:
{code:sql}
ISULL(6 + col, 6)
{code}
Ideally, the above rewrite would be done before the {{ISNULL}} rewrite so we'd
not lose our tag. Then, we can tag the rewritten case EXPR (making something
up):
{code:sql}
CASE WHEN 6 + col {tag: $1} IS NULL THEN ifNull ELSE $1 END
{code}
Or:
{code:sql}
TAGS $1: 6 + col CASE WHEN $1 IS NULL THEN ifNull ELSE $1 END
{code}
The code generation for CASE in the BE would be aware of these tags and would:
* In the same block where the {{CASE expr ...}} is evaluated, evaluate and
store any tagged expressions.
* When performing the {{WHEN}}, {{THEN}} and {{ELSE}} clauses, modify code gen
to refer back to prior saved values.
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