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/PostgreSQL 9.6

63.6. Limitations

GIN assumes that indexable operators are strict. This means that extractValue will not be called at all on a null item value (instead, a placeholder index entry is created automatically), and extractQuery will not be called on a null query value either (instead, the query is presumed to be unsatisfiable). Note however that null key values contained within a non-null composite item or query value are supported.

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https://www.postgresql.org/docs/9.6/static/gin-limit.html