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Fix cost estimate for transitive path #383

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Apr 19, 2021
Merged

Fix cost estimate for transitive path #383

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13 changes: 13 additions & 0 deletions src/engine/TransitivePath.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -150,6 +150,19 @@ size_t TransitivePath::getSizeEstimate() {
if (_rightSideTree != nullptr) {
return _rightSideTree->getSizeEstimate();
}
// Set costs to something very large, so that we never compute the complete
// transitive hull (unless the variables on both sides are not bound in any
// other way, so that the only possible query plan is to compute the complete
// transitive hull).
//
// NOTE: _subtree->getSizeEstimate() is the number of triples of the
// predicate, for which the transitive hull operator (+) is specified. On
// Wikidata, the predicate with the largest blowup when taking the
// transitive hull is wdt:P2789 (connects with). The blowup is then from 90K
// (without +) to 110M (with +), so about 1000 times larger.
if (_leftIsVar && _rightIsVar) {
return _subtree->getSizeEstimate() * 10000;
}
// TODO(Florian): this is not necessarily a good estimator
if (_leftIsVar) {
return _subtree->getSizeEstimate() / _subtree->getMultiplicity(_leftSubCol);
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