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Code Issues

Attempt to avoid hash joining for possible cardinality under-estimations

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This commit is contained in:
Dmitry Yemanov 2024-05-28 12:47:39 +03:00
parent 40d0b4150c
commit 95442bdfff
1 changed files with 7 additions and 1 deletions
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src/jrd/optimizer/InnerJoin.cpp
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@ -191,6 +191,12 @@ void InnerJoin::estimateCost(unsigned position,
// Get the stream cardinality // Get the stream cardinality
const auto streamCardinality = csb->csb_rpt[stream->number].csb_cardinality; const auto streamCardinality = csb->csb_rpt[stream->number].csb_cardinality;
// If the table looks like empty during preparation time, we cannot be sure about
// its real cardinality during execution. So, unless we have some index-based
// filtering applied, let's better be pessimistic and avoid hash joining due to
// likely cardinality under-estimation.
const bool avoidHashJoin = (streamCardinality <= MINIMUM_CARDINALITY && !stream->baseIndexes);
auto currentCardinality = candidate->unique ? auto currentCardinality = candidate->unique ?
MINIMUM_CARDINALITY : streamCardinality * candidate->selectivity; MINIMUM_CARDINALITY : streamCardinality * candidate->selectivity;
auto currentCost = candidate->cost; auto currentCost = candidate->cost;
@ -210,7 +216,7 @@ void InnerJoin::estimateCost(unsigned position,
// Consider whether the current stream can be hash-joined to the prior ones. // Consider whether the current stream can be hash-joined to the prior ones.
// Beware conditional retrievals, this is impossible for them. // Beware conditional retrievals, this is impossible for them.
if (position && !candidate->condition) if (position && !candidate->condition && !avoidHashJoin)
{ {
// Calculate the hashing cost. It consists of the following parts: // Calculate the hashing cost. It consists of the following parts:
// - hashed stream retrieval // - hashed stream retrieval