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Code Issues

A better selectivity estimation for compound index matches in the case of missing statistics

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This commit is contained in:
Dmitry Yemanov 2024-03-31 12:47:34 +03:00
parent b0af516439
commit 5b21261b1c
1 changed files with 63 additions and 63 deletions
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126
src/jrd/optimizer/Retrieval.cpp
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@ -827,6 +827,8 @@ void Retrieval::getInversionCandidates(InversionCandidateList& inversions,
unsigned scope) const
{
const double cardinality = csb->csb_rpt[stream].csb_cardinality;
fb_assert(cardinality);
const double minSelectivity = MIN(MAXIMUM_SELECTIVITY / cardinality, DEFAULT_SELECTIVITY);
// Walk through indexes to calculate selectivity / candidate
MatchedBooleanList matches;
@ -885,6 +887,15 @@ void Retrieval::getInversionCandidates(InversionCandidateList& inversions,
}
}
auto selectivity = idx->idx_rpt[j].idx_selectivity;
const auto useDefaultSelectivity = (selectivity <= 0);
// When the index selectivity is zero then the statement is prepared
// on an empty table or the statistics aren't updated. So assume every
// match to represent 1/10 of the maximum selectivity.
if (useDefaultSelectivity)
selectivity = MAX(scratch.selectivity * DEFAULT_SELECTIVITY, minSelectivity);
if (segment.scanType == segmentScanList)
{
if (listCount) // we cannot have more than one list matched to an index
@ -907,10 +918,10 @@ void Retrieval::getInversionCandidates(InversionCandidateList& inversions,
// This is a perfect usable segment thus update root selectivity
scratch.lowerCount++;
scratch.upperCount++;
scratch.selectivity = idx->idx_rpt[j].idx_selectivity;
scratch.nonFullMatchedSegments = idx->idx_count - (j + 1);
// Add matches for this segment to the main matches list
matches.join(segment.matches);
scratch.selectivity = selectivity;
// An equality scan for any unique index cannot retrieve more
// than one row. The same is true for an equivalence scan for
@ -931,6 +942,12 @@ void Retrieval::getInversionCandidates(InversionCandidateList& inversions,
// so we can stop looking further, because this is the best
// one we can get.
unique = true;
// If selectivity is assumed, a better guess for the unique match
// would be 1 / cardinality
if (useDefaultSelectivity)
scratch.selectivity = minSelectivity;
break;
}
@ -942,58 +959,53 @@ void Retrieval::getInversionCandidates(InversionCandidateList& inversions,
}
else
{
// This is our last segment that we can use,
// estimate the selectivity
auto selectivity = scratch.selectivity;
double factor = 1;
switch (segment.scanType)
{
case segmentScanBetween:
scratch.lowerCount++;
scratch.upperCount++;
selectivity = idx->idx_rpt[j].idx_selectivity;
factor = REDUCE_SELECTIVITY_FACTOR_BETWEEN;
break;
case segmentScanLess:
scratch.upperCount++;
selectivity = idx->idx_rpt[j].idx_selectivity;
factor = REDUCE_SELECTIVITY_FACTOR_LESS;
break;
case segmentScanGreater:
scratch.lowerCount++;
selectivity = idx->idx_rpt[j].idx_selectivity;
factor = REDUCE_SELECTIVITY_FACTOR_GREATER;
break;
case segmentScanStarting:
case segmentScanEqual:
case segmentScanEquivalent:
scratch.lowerCount++;
scratch.upperCount++;
selectivity = idx->idx_rpt[j].idx_selectivity;
factor = REDUCE_SELECTIVITY_FACTOR_STARTING;
break;
default:
fb_assert(segment.scanType == segmentScanNone);
break;
}
// Adjust the compound selectivity using the reduce factor.
// It should be better than the previous segment but worse
// than a full match.
const double diffSelectivity = scratch.selectivity - selectivity;
selectivity += (diffSelectivity * factor);
fb_assert(selectivity <= scratch.selectivity);
scratch.selectivity = selectivity;
if (segment.scanType != segmentScanNone)
{
matches.join(segment.matches);
// This is our last segment that we can use,
// estimate the selectivity
double factor = 1;
switch (segment.scanType)
{
case segmentScanBetween:
scratch.lowerCount++;
scratch.upperCount++;
factor = REDUCE_SELECTIVITY_FACTOR_BETWEEN;
break;
case segmentScanLess:
scratch.upperCount++;
factor = REDUCE_SELECTIVITY_FACTOR_LESS;
break;
case segmentScanGreater:
scratch.lowerCount++;
factor = REDUCE_SELECTIVITY_FACTOR_GREATER;
break;
case segmentScanStarting:
case segmentScanEqual:
case segmentScanEquivalent:
scratch.lowerCount++;
scratch.upperCount++;
factor = REDUCE_SELECTIVITY_FACTOR_STARTING;
break;
default:
fb_assert(false);
break;
}
// Adjust the compound selectivity using the reduce factor.
// It should be better than the previous segment but worse
// than a full match.
const double diffSelectivity = scratch.selectivity - selectivity;
selectivity += (diffSelectivity * factor);
fb_assert(selectivity <= scratch.selectivity);
scratch.selectivity = selectivity;
scratch.nonFullMatchedSegments = idx->idx_count - j;
matches.join(segment.matches);
}
break;
@ -1003,19 +1015,7 @@ void Retrieval::getInversionCandidates(InversionCandidateList& inversions,
if (scratch.scopeCandidate)
{
double selectivity = scratch.selectivity;
// When the index selectivity is zero then the statement is prepared
// on an empty table or the statistics aren't updated. The priorly
// calculated selectivity is meaningless in this case. So instead:
// - for an unique match, 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 utilized by the optimizer.
if (idx->idx_selectivity <= 0)
{
selectivity = unique ?
MIN(MAXIMUM_SELECTIVITY / cardinality, DEFAULT_SELECTIVITY) :
DEFAULT_SELECTIVITY;
}
fb_assert(selectivity);
// Calculate the cost (only index pages) for this index
auto cost = DEFAULT_INDEX_COST + selectivity * scratch.cardinality;