Defines which fields should be considered for result collapsing.
It consist of sequences of metadata class characters separated by commas. The characters $ and # may be used as metadata class characters and represent document summarisable text and document URL respectively. Commas are used to separate columns which can be selected at query time using the appropriate query processor option.
Results are collapsed by their summarisable text by default:
Generate 2 signatures per document: Column 0 is the normal document signature and column 1 is a signature derived from the concatenation of metadata fields t and a in that order:
using the "t" metadata
When "t" is used as the collapsing field, collapsing will apply for the actual value of the "t" metadata,
which may be different from the document title. The best way to understand the differences is to display
both the result title, and the value of the "t" metadata next to each other (Using the JSON
or XML endpoint and the
parameters). Usually, "t" will be multi-valued as the default metadata mapping maps it to different
title fields and HTML h1 .. h4 headers.