PostgreSQL 9.2.1 Documentation
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# 55.5. 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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