Difference between BooleanClause.Occur.Must and BooleanClause.Occur.SHOULD in lucene

Question

Can anyone explain the difference between the BooleanClause.Occur.Must and BooleanClause.Occur.SHOULD in lucene in BooleanQuery with an example?

Answer 1

BooleanClause.Occur.SHOULD means that the clause is optional, whereas BooleanClause.Occur.Must means that the clause is compulsory.

However, if a boolean query only has optional clauses, at least one clause must match for a document to appear in the results.

For better control over what documents match a BooleanQuery, there is also a minimumShouldMatch parameter which lets you tell Lucene that at least minimumShouldMatch BooleanClause.Occur.SHOULD clauses must match for a document to appear in the results.

Answer 2

I will try to explain using an example:

Let's assume that there are two clauses: Clause A and Clause B. Now the effect of the BooleanClause.Occur will be as follows:

• In first case, both clause A and B have BooleanClause.Occur.Should flag set. This will imply that even if one of the clause is satisfied (A or B), then the document will be a hit.

• In second case, clause A has BooleanClause.Occur.Must flag set and clause B has BooleanClause.Occur.Should flag set.

  In this case, a document will be a hit when it "will" satisfy clause A. Whether this document satisfies clause B or not will have no effect on it being a hit.

  But if the document does not satisfies clause A, then no matter whether it satisfies clause B or not, it will not be a hit.

• In third case, both clause A and clause B have the BooleanClause.Occur.Must flag set.

  In this case, a document will be a hit, only when it will satisfy "both" the clauses. If it will fail to satisfy even one of the clause, then it will not be a hit.

Answer 3

BooleanClause.Occur.Must stands for a Mandatory clause. The Clause should be met for the result to be returned. Basically AND.

BooleanClause.Occur.SHOULD stands for the optional clause and would behave like an OR