Filter expressions are attached to queries, or, more formally, to its corpus keys, and serve to restrict the search to only the part of the corpus that matches the expression. In both form and function, they are a simpler version of a where clause's search condition in ANSI SQL, see §7.6.
An expression operates on the metadata attached to documents that are indexed
in Vectara. Because this metadata can be associated to
either the entire document, or to specific parts within it, the scope must
be explicitly specified for every metadata reference in the expression. Valid
part., for document and part-level metadata,
The expression below selects customer reviews in German with better than a 3-star rating. Note that while there is a single rating for the entire document, the detected language is set at the part level.
doc.rating > 3.0 and part.lang = 'deu'
lang metadata tag used in this example is detected and set automatically
by the platform at indexing time. It's set at the part level for accuracy,
because a single document may contain content in multiple languages.
More complicated expressions are possible, such as the one below, which checks for documents with a publication date in 2021.
1609459200 < doc.pub_epoch and doc.pub_epoch < 1640995200
pub_epoch stores the date in epoch time.
You can find a full list of supported syntax on the Functions and Operators page.