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Code Issues

Better selectivity assumption for compound indices on empty tables

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This commit is contained in:
Dmitry Yemanov 2023-09-11 20:19:21 +03:00
parent b3c54800f3
commit 29b269b129
1 changed files with 12 additions and 10 deletions
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22
src/jrd/optimizer/Retrieval.cpp
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@ -898,6 +898,17 @@ void Retrieval::getInversionCandidates(InversionCandidateList& inversions,
// Add matches for this segment to the main matches list
matches.join(segment.matches);
// When selectivity is zero the statement is prepared on an
// empty table or the statistics aren't updated.
// For an unique index, estimate the selectivity via the stream cardinality.
// For a non-unique one, assume 1/10 of the maximum selectivity, so that
// at least some indexes could be chosen by the optimizer.
if (scratch.selectivity <= 0)
{
scratch.selectivity = (unique && cardinality > MINIMUM_CARDINALITY) ?
1 / cardinality : DEFAULT_SELECTIVITY;
}
// An equality scan for any unique index cannot retrieve more
// than one row. The same is true for an equivalence scan for
// any primary index.
@ -988,16 +999,8 @@ void Retrieval::getInversionCandidates(InversionCandidateList& inversions,
if (scratch.scopeCandidate)
{
// When selectivity is zero the statement is prepared on an
// empty table or the statistics aren't updated.
// For an unique index, estimate the selectivity via the stream cardinality.
// For a non-unique one, assume 1/10 of the maximum selectivity, so that
// at least some indexes could be chosen by the optimizer.
double selectivity = scratch.selectivity;
if (selectivity <= 0)
selectivity = unique ? 1 / cardinality : DEFAULT_SELECTIVITY;
// Calculate the cost (only index pages) for this index
auto cost = DEFAULT_INDEX_COST + selectivity * scratch.cardinality;
@ -1007,8 +1010,7 @@ void Retrieval::getInversionCandidates(InversionCandidateList& inversions,
selectivity *= listCount;
selectivity = MIN(selectivity, maxSelectivity);
const auto rootScanCost = DEFAULT_INDEX_COST * listCount +
scratch.cardinality * selectivity;
const auto rootScanCost = cost * listCount;
const auto siblingScanCost = DEFAULT_INDEX_COST +
scratch.cardinality * maxSelectivity * (listCount - 1) / (listCount + 1);