PostgreSQL 8.4.22 Documentation | ||||
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GIN doesn't support full
index scans. The reason for this is that extractValue
is allowed to return zero keys, as
for example might happen with an empty string or empty array. In
such a case the indexed value will be unrepresented in the index.
It is therefore impossible for GIN to guarantee that a scan of the index can
find every row in the table.
Because of this limitation, when extractQuery
returns nkeys
= 0 to indicate that all values match the query,
GIN will emit an error. (If
there are multiple ANDed indexable operators in the query, this
happens only if they all return zero for nkeys.)
It is possible for an operator class to circumvent the
restriction against full index scan. To do that, extractValue
must return at least one (possibly
dummy) key for every indexed value, and extractQuery
must convert an unrestricted
search into a partial-match query that will scan the whole index.
This is inefficient but might be necessary to avoid corner-case
failures with operators such as LIKE or
subset inclusion.
GIN assumes that indexable
operators are strict. This means that extractValue
will not be called at all on a
NULL value (so the value will go unindexed), and extractQuery
will not be called on a NULL
comparison value either (instead, the query is presumed to be
unmatchable).
A possibly more serious limitation is that GIN cannot handle NULL keys — for example, an array containing a NULL cannot be handled except by ignoring the NULL.