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Don't use zero cost estimate for cached subtrees #1783

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merged 2 commits into from
Feb 9, 2025
Merged

Don't use zero cost estimate for cached subtrees #1783

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612a8df
Don't use zero cost for cached subtrees
Feb 9, 2025
bed12e4
Address Robin's comment and improve parameter name
Feb 9, 2025
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9 changes: 7 additions & 2 deletions src/engine/QueryExecutionTree.cpp
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Expand Up @@ -77,10 +77,15 @@ QueryExecutionTree::selectedVariablesToColumnIndices(

// _____________________________________________________________________________
size_t QueryExecutionTree::getCostEstimate() {
if (cachedResult_) {
// result is pinned in cache. Nothing to compute
// If the result is cached and `zero-cost-estimate-for-cached-subtrees` is set
// to `true`, we set the cost estimate to zero.
if (cachedResult_ &&
RuntimeParameters().get<"zero-cost-estimate-for-cached-subtree">()) {
return 0;
}

// Otherwise, we return the cost estimate of the root operation. For index
// scans, we assume one unit of work per result row.
if (getRootOperation()->isIndexScanWithNumVariables(1)) {
return getSizeEstimate();
} else {
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3 changes: 3 additions & 0 deletions src/global/RuntimeParameters.h
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Expand Up @@ -60,6 +60,9 @@ inline auto& RuntimeParameters() {
// its size estimate will be the size of the block divided by this
// value.
SizeT<"small-index-scan-size-estimate-divisor">{5},
// Determines whether the cost estimate for a cached subtree should be
// set to zero in query planning.
Bool<"zero-cost-estimate-for-cached-subtree">{false},
};
}();
return params;
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17 changes: 15 additions & 2 deletions test/OperationTest.cpp
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Expand Up @@ -242,15 +242,28 @@ TEST(OperationTest, estimatesForCachedResults) {

[[maybe_unused]] auto res = qet->getResult();
}
// The result is now cached inside the static execution context, if we create
// the same operation again, the cost estimate is 0. The size estimate doesn't
// The result is now cached inside the static execution context. If we create
// the same operation again and `zero-cost-estimate-for-cached-subtrees` is
// set to `true`, the cost estimate should be zero. The size estimate does not
// change (see the `getCostEstimate` function for details on why).
{
auto restoreWhenScopeEnds =
setRuntimeParameterForTest<"zero-cost-estimate-for-cached-subtree">(
true);
auto qet = makeQet();
EXPECT_EQ(qet->getCacheKey(), qet->getRootOperation()->getCacheKey());
EXPECT_EQ(qet->getSizeEstimate(), 24u);
EXPECT_EQ(qet->getCostEstimate(), 0u);
}
{
auto restoreWhenScopeEnds =
setRuntimeParameterForTest<"zero-cost-estimate-for-cached-subtree">(
false);
auto qet = makeQet();
EXPECT_EQ(qet->getCacheKey(), qet->getRootOperation()->getCacheKey());
EXPECT_EQ(qet->getSizeEstimate(), 24u);
EXPECT_EQ(qet->getCostEstimate(), 210u);
}
}

// ________________________________________________
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