Collapse and Expand Results

Collapsing Query Parser Options

The CollapsingQParser accepts the following local parameters:

Sample Usage Syntax

Collapse on group_field selecting the document in each group with the highest scoring document:

fq={!collapse field=group_field}

Collapse on group_field selecting the document in each group with the minimum value of numeric_field:

fq={!collapse field=group_field min=numeric_field}

Collapse on group_field selecting the document in each group with the maximum value of numeric_field:

fq={!collapse field=group_field max=numeric_field}

Collapse on group_field selecting the document in each group with the maximum value of a function. Note that the cscore() function can be used with the min/max options to use the score of the current document being collapsed.

fq={!collapse field=group_field max=sum(cscore(),numeric_field)}

Collapse on group_field with a null policy so that all docs that do not have a value in the group_field will be treated as a single group. For each group, the selected document will be based first on a numeric_field, but ties will be broken by score:

fq={!collapse field=group_field nullPolicy=collapse sort='numeric_field asc, score desc'}

Collapse on group_field with a hint to use the top level field cache:

fq={!collapse field=group_field hint=top_fc}

Collapse with custom cost which defaults to 100

fq={!collapse cost=1000 field=group_field}

Block Collapsing

When collapsing on the _root_ field, using nullPolicy=expand or nullPolicy=ignore, the Collapsing Query Parser can take advantage of the fact that all docs with identical field values are adjacent to each other in the index in a single "block" of nested documents. This allows the collapsing logic to be much faster and more memory efficient.

The default collapsing logic must keep track of all group head documents — for all groups encountered so far — until it has evaluated all documents, because each document it considers may become the new group head of any group.

When collapsing on the _root_ field however, the logic knows that as it scans over the index, it will never encounter any new documents in a group that it has previously processed.

This more efficient logic can also be used with other collapseField values, via the hint=block local param. This can be useful when you have deeply nested documents and you’d like to collapse on a field that does not contain identical values for all documents with a common _root_ but is a unique and identical value for sets of contiguous documents under a common _root_. For example: searching for "grand child" documents and collapsing on a field that is unique per "child document"