Insertion into a GIN index can be slow due to the likelihood of many keys being inserted for each item. So, for bulk insertions into a table it is advisable to drop the GIN index and recreate it after finishing bulk insertion.
As of PostgreSQL 8.4, this advice is less necessary since delayed indexing is used (see Section 64.4.1 for details). But for very large updates it may still be best to drop and recreate the index.
Build time for a GIN index is very sensitive to
the maintenance_work_mem
setting; it doesn't pay to
skimp on work memory during index creation.
During a series of insertions into an existing GIN
index that has fastupdate
enabled, the system will clean up
the pending-entry list whenever the list grows larger than
gin_pending_list_limit
. To avoid fluctuations in observed
response time, it's desirable to have pending-list cleanup occur in the
background (i.e., via autovacuum). Foreground cleanup operations
can be avoided by increasing gin_pending_list_limit
or making autovacuum more aggressive.
However, enlarging the threshold of the cleanup operation means that
if a foreground cleanup does occur, it will take even longer.
gin_pending_list_limit
can be overridden for individual
GIN indexes by changing storage parameters, and which allows each
GIN index to have its own cleanup threshold.
For example, it's possible to increase the threshold only for the GIN
index which can be updated heavily, and decrease it otherwise.
The primary goal of developing GIN indexes was to create support for highly scalable full-text search in PostgreSQL, and there are often situations when a full-text search returns a very large set of results. Moreover, this often happens when the query contains very frequent words, so that the large result set is not even useful. Since reading many tuples from the disk and sorting them could take a lot of time, this is unacceptable for production. (Note that the index search itself is very fast.)
To facilitate controlled execution of such queries,
GIN has a configurable soft upper limit on the
number of rows returned: the
gin_fuzzy_search_limit
configuration parameter.
It is set to 0 (meaning no limit) by default.
If a non-zero limit is set, then the returned set is a subset of
the whole result set, chosen at random.
“Soft” means that the actual number of returned results could differ somewhat from the specified limit, depending on the query and the quality of the system's random number generator.
From experience, values in the thousands (e.g., 5000 — 20000) work well.