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46 lines
1.9 KiB
Diff
46 lines
1.9 KiB
Diff
commit d4edb0510ec1189f65850bb47977e94ed98b1f71
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Author: Sergei Petrunia <psergey@askmonty.org>
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Date: Wed Nov 13 18:53:59 2019 +0300
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MDEV-20646: 10.3.18 is slower than 10.3.17
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Fix incorrect change introduced in the fix for MDEV-20109.
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The patch tried to compute a more precise estimate for the record_count
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value in SJ-Materialization-Scan strategy (in
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Sj_materialization_picker::check_qep). However the new formula is worse
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as it produces extremely optimistic results in common cases where
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SJ-Materialization-Scan should be used)
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The old formula produces pessimistic results in cases when Sj-Materialization-
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Scan is unlikely to be a good choice anyway. So, the old behavior is better.
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diff --git a/sql/opt_subselect.cc b/sql/opt_subselect.cc
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index aeafc13998a..a8afd952a4d 100644
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--- a/sql/opt_subselect.cc
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+++ b/sql/opt_subselect.cc
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@@ -3029,7 +3029,22 @@ bool Sj_materialization_picker::check_qep(JOIN *join,
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*strategy= SJ_OPT_MATERIALIZE_SCAN;
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*read_time= prefix_cost;
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- *record_count= prefix_rec_count / mat_info->rows_with_duplicates;
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+ /*
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+ Note: the next line means we did not remove the subquery's fanout from
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+ *record_count. It needs to be removed, as the join prefix is
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+
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+ ntX SJM-SCAN(it1 ... itN) | (ot1 ... otN) ...
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+
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+ here, the SJM-SCAN may have introduced subquery's fanout (duplicate rows,
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+ rows that don't have matches in ot1_i). All this fanout is gone after
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+ table otN (or earlier) but taking it into account is hard.
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+
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+ Some consolation here is that SJM-Scan strategy is applicable when the
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+ subquery is smaller than tables otX. If the subquery has large cardinality,
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+ we can greatly overestimate *record_count here, but it doesn't matter as
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+ SJ-Materialization-Lookup is a better strategy anyway.
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+ */
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+ *record_count= prefix_rec_count;
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*handled_fanout= mat_nest->sj_inner_tables;
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return TRUE;
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}
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