Dmitry Lychagin created ASTERIXDB-2441:
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Summary: Change how column aliases are inlined into order by and
limit clauses
Key: ASTERIXDB-2441
URL: https://issues.apache.org/jira/browse/ASTERIXDB-2441
Project: Apache AsterixDB
Issue Type: Improvement
Reporter: Dmitry Lychagin
Assignee: Dmitry Lychagin
Currently when a column alias is referenced by an ORDER BY or a LIMIT clause we
copy the column alias expression into that clause during query rewriting (
SqlppQueryRewriter.inlineColumnAlias()). This may lead to suboptimal query
plans when the copied expression is complex and cannot be factored out by the
optimizer's ExtractCommonExpressionsRule (for example AGGREGATE_FN(DISTINCT
...). In those cases the expression will be executed twice an runtime.
A better approach is to introduce a variable for that expression (LET clause)
and refer to that variable from SELECT and ORDER BY/LIMIT clauses. This way the
optimizer can decide whether this LET clause should be inlined or not.
For example.
SELECT f(x) AS y FROM ... ORDER BY y
Is currently rewritten into
SELECT f(x) AS y FROM ... ORDER BY f(x)
With the new approach it will be rewritten into
SELECT z AS y FROM ... LET z = f(x) ORDER BY z
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