txwei commented on code in PR #16801:
URL: https://github.com/apache/lucene/pull/16801#discussion_r4300672434
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lucene/core/src/java/org/apache/lucene/search/TermInSetQuery.java:
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@@ -164,6 +164,27 @@ public long getTermsCount() {
return termData.size();
}
+ @Override
+ public long estimateCost(Terms terms) throws IOException {
+ // Estimate the cost. If the MTQ can provide its term count, we can do a
better job
+ // estimating.
+ // Cost estimation reasoning is:
+ // Since we know how many query terms there are...
+ // a. Assume every query term matches at least one document
(queryTermsCount).
+ // b. Determine the total number of docs beyond the first one for each
term.
+ // That count provides a ceiling on the number of extra docs that
could match beyond
+ // that first one. (We omit the first since it's already been counted
in 2a).
+ // For id fields (i.e. docFreq = 1 for every term), the estimated cost
will be equal
+ // to the number of query terms.
+ // See: LUCENE-10207
+ long potentialExtraCost = terms.getSumDocFreq();
+ final long indexedTermCount = terms.size();
+ if (indexedTermCount != -1) {
+ potentialExtraCost -= indexedTermCount;
+ }
+ return termData.size() + potentialExtraCost;
Review Comment:
can we cap by `terms.getrDocCount()`? If `TermInSetQuery` has more terms
than the segment's dictionary, or if this is a multi valued field with
`sumDocFreq` > `docCount`, we could provide a tighter estimation here
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