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We optimize plain count(*) queries on Iceberg tables the following way:
AGGREGATE
COUNT(*)
|
UNION ALL
/ \
/ \
/ \
SCAN all ANTI JOIN
datafiles / \
without / \
deletes SCAN SCAN
datafiles deletes
||
rewrite
||
\/
ArithmethicExpr: LHS + RHS
/ \
/ \
/ \
record_count AGGREGATE
of all COUNT(*)
datafiles |
without ANTI JOIN
deletes / \
/ \
SCAN SCAN
datafiles deletes
This optimization consists of two parts:
1 Rewriting count(*) expression to count(*) + "record_count" (of data
files without deletes)
2 In IcebergScanPlanner we only need to consruct the right side of
the original UNION ALL operator, i.e.:
ANTI JOIN
/ \
/ \
SCAN SCAN
datafiles deletes
SelectStmt decides whether we can do the count(*) optimization, and if
so, does the following:
1: SelectStmt sets 'TotalRecordsNumV2' in the analyzer, then during the
expression rewrite phase the CountStarToConstRule rewrites the
count(*) to count(*) + record_count
2: SelectStmt sets "OptimizeCountStarForIcebergV2" in the query context
then IcebergScanPlanner creates plan accordingly.
This mechanism works for simple queries, but can turn on count(*)
optimization in IcebergScanPlanner for all Iceberg V2 tables in complex
queries. Even if only one subquery enables count(*) optimization during
analysis.
With this patch the followings change:
1: We introduce IcebergV2CountStarAccumulator which we use instead of
the ArithmethicExpr. So after rewrite we still know if count(*)
optimization should be enabled for the planner.
2: Instead of using the query context, we pass the information to the
IcebergScanPlanner via the TableRef object.
Testing
* e2e tests
Change-Id: I1940031298eb634aa82c3d32bbbf16bce8eaf874
Reviewed-on: http://gerrit.cloudera.org:8080/23705
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
Reviewed-by: Zoltan Borok-Nagy <boroknagyz@cloudera.com>