Help:CirrusSearch/Logical operators

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CirrusSearch currently does not support classic boolean searching, and the logical operators AND and OR should be used with great care, if at all.

Negation and parentheses

CirrusSearch does support several ways of indicating negation. The following queries are all equivalent: -dog (minus sign), !dog (exclamation point), and NOT dog (NOT operator).

CirrusSearch does not support parentheses, and they are removed from the query.

Lucene, MUST, and SHOULD

CirrusSearch is built on top of Elasticsearch, which in turn is built on Lucene. Our Lucene implementation does not support the classic boolean AND or OR operators, though it does offer those keywords as binary operators.

Instead Lucene converts AND and OR to a different formalism—unary MUST and SHOULD operators—giving results that sometimes mimic the expected boolean results, but which can also be very divergent from them. (Note that CirrusSearch does not currently support MUST or SHOULD operators in user queries. They are used here only to demonstrate the internal workings of Lucene.)

In Lucene, MUST indicates that a search term is required and must be present in any results. So, a query like MUST dog would only return results that contain some form of dog in them (note that this would also be equivalent to just searching for dog).

On the other hand, SHOULD terms are optional but should be present if possible; while they are not strictly required, they do effect ranking. So MUST dog SHOULD cat would require dog in every result, but would generally rank those that also contain cat as better matches.

The one exception to SHOULD terms being optional is that if there are zero MUST terms, then at least one SHOULD term would be present in each result. Thus, SHOULD dog SHOULD cat SHOULD fish would actually give results that have at least one of dog, cat, or fish present—though any results with all three would generally rank higher.

Classic boolean search often has an implicit AND, meaning that any query terms without an explicit boolean operator between them are assumed to have an AND between them. In Lucene, any query term without an explicit MUST or SHOULD is assumed to have an implicit MUST applied to it.

Converting AND and OR

Lucene converts AND and OR to MUST and SHOULD in a way that sometimes gives the expected results, but often leads to very unexpected results.

When Lucene encounters AND, it applies MUST to the terms before and after the AND. When it encounters OR, it applies SHOULD to the terms before and after the OR. The query is processed left to right, and later AND or OR operators override earlier ones (see examples below).

This effectively gives an unusual "backward order precedence" to the operators, and the results can be quite unexpected compared to classic boolean searching.

Examples that go wrong

Below are some worked examples where the conversion from AND/OR to MUST/SHOULD gives divergent results from the expectations of classic boolean operators.

  • blue OR red AND green
    • convert OR to SHOULD before and after, giving:
  • SHOULD blue SHOULD red AND green
    • convert AND to MUST before and after (in this case overriding the previously applied SHOULD), giving:
  • SHOULD blue MUST red MUST green
    • The result set is thus the same as red green, with blue being optional (and only affecting ranking).
  • blue OR red green
    • convert OR to SHOULD before and after, giving:
  • SHOULD blue SHOULD red green
    • apply an implicit MUST to any term without an explicit MUST or SHOULD, giving:
  • SHOULD blue SHOULD red MUST green
    • In a classic boolean system with implicit AND, we would expect that blue OR red AND green and blue OR red green to be the same, but compare this to the example above to see the difference—only green is required here, while red and green are both required above.
  • blue AND red OR green
    • convert AND to MUST before and after, giving:
  • MUST blue MUST red OR green
    • convert OR to SHOULD before and after, giving:
  • MUST blue SHOULD red SHOULD green
    • The result set is thus the same as simply searching for blue, with red and green only affecting ranking. This also means that if there are zero documents with either red or green in them, you will get the same results searching for blue AND red OR green as you would for just searching for blue, which is not what you would expect from a classic boolean system.

In general, mixing OR with AND, including implicit AND in one query gives results that are unintuitive in a classic boolean framework. It can also be very difficult to detect these cases where the boolean logic goes awry, unless you already know exactly how many documents contain each possible positive and negative combination of your query terms.

Common use cases

If you have no explicit operators, then the boolean default is AND and the Lucene default is MUST, which are equivalent if they are the only operators present in the query:

  • blue red green — user intent: all three terms must be present in any results
  • blue AND red AND green — explicit classic boolean query: all three terms must be present in any results
  • MUST blue MUST red MUST green — Lucene interpretation: all three terms must be present in any results

However, since MUST is implicit, nothing is gained by making it explicit by using AND, other than the potential for later boolean confusion.

If the only operator in the query is OR—crucially meaning that there is no implicit AND, then it is the same as everything having a SHOULD (recall that if a query has SHOULD terms but no MUST terms, than at least one of the SHOULD terms will be present in any result):

  • blue OR red OR green — classic boolean query: at least one of the three terms must be present in any results
  • SHOULD blue SHOULD red SHOULD green — Lucene interpretation: at least one of the three terms must be present in any results

Be very careful with implicit AND/MUST! In the example above, blue OR red green the implicit MUST applied to green means that neither blue nor red are strictly required to be in the results.

Booleans, keywords, and prefixes

AND and OR do not interact predictably with special keywords (like insource: or hastemplate:) or with namespaces (like Talk: or User:) and probably should not be used in conjunction with either.

Future plans

Of course, the Search Platform team is not very happy with this state of affairs.

In the short term we are creating this document and updating the Help:CirrusSearch documentation to reflect the reality of our current system.

Longer term, we plan to implement a new layer in CirrusSearch that will properly construct a Lucene MUST/SHOULD query that is equivalent to a given classic boolean query, including proper support for parentheses and return the expected results. (It is possible to specify in Lucene that at least one of a set of query terms or clauses is a required to match, which is equivalent to a boolean OR; requiring that all of a set of query terms or clauses match is the same as a boolean AND.)

Beyond that, we may also make explicit the MUST and SHOULD operators, possibly using the unary syntax shown in this document, but also possibly using some other syntax, as yet to be determined.

Further reading

  • BooleanQuerySyntax — a summary of a mailing list discussion about the problem, going back to 2005, with a link to a bug report on the problem from 2003. (The 2003 bug was closed in 2009, and claims there is a different Lucene query parser that does the right thing with boolean queries, but we don't have access to it in CirrusSearch.)
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