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Combining Queries
editCombining Queries
editIn Combining Filters we discussed how to use the bool
filter to combine
multiple filter clauses with and
, or
, and not
logic. In query land, the
bool
query does a similar job but with one important difference.
Filters make a binary decision: should this document be included in the results list or not? Queries, however, are more subtle. They decide not only whether to include a document, but also how relevant that document is.
Like the filter equivalent, the bool
query accepts multiple query clauses
under the must
, must_not
, and should
parameters. For instance:
GET /my_index/my_type/_search { "query": { "bool": { "must": { "match": { "title": "quick" }}, "must_not": { "match": { "title": "lazy" }}, "should": [ { "match": { "title": "brown" }}, { "match": { "title": "dog" }} ] } } }
The results from the preceding query include any document whose title
field
contains the term quick
, except for those that also contain lazy
. So
far, this is pretty similar to how the bool
filter works.
The difference comes in with the two should
clauses, which say that: a document
is not required to contain either brown
or dog
, but if it does, then
it should be considered more relevant:
{ "hits": [ { "_id": "3", "_score": 0.70134366, "_source": { "title": "The quick brown fox jumps over the quick dog" } }, { "_id": "1", "_score": 0.3312608, "_source": { "title": "The quick brown fox" } } ] }
Score Calculation
editThe bool
query calculates the relevance _score
for each document by adding
together the _score
from all of the matching must
and should
clauses,
and then dividing by the total number of must
and should
clauses.
The must_not
clauses do not affect the score; their only purpose is to
exclude documents that might otherwise have been included.
Controlling Precision
editAll the must
clauses must match, and all the must_not
clauses must not
match, but how many should
clauses should match? By default, none of the should
clauses are required to match, with one
exception: if there are no must
clauses, then at least one should
clause
must match.
Just as we can control the precision of the match
query,
we can control how many should
clauses need to match by using the
minimum_should_match
parameter, either as an absolute number or as a
percentage:
GET /my_index/my_type/_search { "query": { "bool": { "should": [ { "match": { "title": "brown" }}, { "match": { "title": "fox" }}, { "match": { "title": "dog" }} ], "minimum_should_match": 2 } } }
The results would include only documents whose title
field contains "brown"
AND "fox"
, "brown" AND "dog"
, or "fox" AND "dog"
. If a document contains
all three, it would be considered more relevant than those that contain
just two of the three.