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60.4.1. GIN Fast Update Technique

60.4.2. Partial Match Algorithm

Updating aGIN index tends to be slow because of the intrinsic nature of inverted indexes: inserting or updating one heap row can cause many inserts into the index (one for each key extracted from the indexed item). As ofPostgreSQL 8.4,GIN is capable of postponing much of this work by inserting new tuples into a temporary, unsorted list of pending entries. When the table is vacuumed or autoanalyzed, or whengin_clean_pending_list function is called, or if the pending list becomes larger thangin_pending_list_limit, the entries are moved to the mainGIN data structure using the same bulk insert techniques used during initial index creation. This greatly improvesGIN index update speed, even counting the additional vacuum overhead. Moreover the overhead work can be done by a background process instead of in foreground query processing.

The main disadvantage of this approach is that searches must scan the list of pending entries in addition to searching the regular index, and so a large list of pending entries will slow searches significantly. Another disadvantage is that, while most updates are fast, an update that causes the pending list to become“too large” will incur an immediate cleanup cycle and thus be much slower than other updates. Proper use of autovacuum can minimize both of these problems.

If consistent response time is more important than update speed, use of pending entries can be disabled by turning off thefastupdate storage parameter for aGIN index. SeeCREATE INDEX for details.

GIN can support“partial match” queries, in which the query does not determine an exact match for one or more keys, but the possible matches fall within a reasonably narrow range of key values (within the key sorting order determined by thecompare support method). TheextractQuery method, instead of returning a key value to be matched exactly, returns a key value that is the lower bound of the range to be searched, and sets thepmatch flag true. The key range is then scanned using thecomparePartial method.comparePartial must return zero for a matching index key, less than zero for a non-match that is still within the range to be searched, or greater than zero if the index key is past the range that could match.