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Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly. You can read PostgreSQL as Postgres-XC except for version number, which is specific to each product.
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.