index GIN GIN stands for Generalized Inverted Index. It is an index structure storing a set of (key, posting list) pairs, where a posting list is a set of rows in which the key occurs. Each indexed value can contain many keys, so the same row ID can appear in multiple posting lists. It is generalized in the sense that a GIN index does not need to be aware of the operation that it accelerates. Instead, it uses custom strategies defined for particular data types. One advantage of GIN is that it allows the development of custom data types with the appropriate access methods, by an expert in the domain of the data type, rather than a database expert. This is much the same advantage as using GiST. The GIN implementation in PostgreSQL is primarily maintained by Teodor Sigaev and Oleg Bartunov. There is more information about GIN on their website. The GIN interface has a high level of abstraction, requiring the access method implementer only to implement the semantics of the data type being accessed. The GIN layer itself takes care of concurrency, logging and searching the tree structure. All it takes to get a GIN access method working is to implement four (or five) user-defined methods, which define the behavior of keys in the tree and the relationships between keys, indexed values, and indexable queries. In short, GIN combines extensibility with generality, code reuse, and a clean interface. The four methods that an operator class for GIN must provide are: int compare(Datum a, Datum b) Compares keys (not indexed values!) and returns an integer less than zero, zero, or greater than zero, indicating whether the first key is less than, equal to, or greater than the second. Datum *extractValue(Datum inputValue, int32 *nkeys) Returns an array of keys given a value to be indexed. The number of returned keys must be stored into *nkeys. Datum *extractQuery(Datum query, int32 *nkeys, StrategyNumber n, bool **pmatch) Returns an array of keys given a value to be queried; that is, query is the value on the right-hand side of an indexable operator whose left-hand side is the indexed column. n is the strategy number of the operator within the operator class (see ). Often, extractQuery will need to consult n to determine the data type of query and the key values that need to be extracted. The number of returned keys must be stored into *nkeys. If the query contains no keys then extractQuery should store 0 or -1 into *nkeys, depending on the semantics of the operator. 0 means that every value matches the query and a sequential scan should be produced. -1 means nothing can match the query. pmatch is an output argument for use when partial match is supported. To use it, extractQuery must allocate an array of *nkeys booleans and store its address at *pmatch. Each element of the array should be set to TRUE if the corresponding key requires partial match, FALSE if not. If *pmatch is set to NULL then GIN assumes partial match is not required. The variable is initialized to NULL before call, so this argument can simply be ignored by operator classes that do not support partial match. bool consistent(bool check[], StrategyNumber n, Datum query, bool *recheck) Returns TRUE if the indexed value satisfies the query operator with strategy number n (or might satisfy, if the recheck indication is returned). The check array has the same length as the number of keys previously returned by extractQuery for this query. Each element of the check array is TRUE if the indexed value contains the corresponding query key, ie, if (check[i] == TRUE) the i-th key of the extractQuery result array is present in the indexed value. The original query datum (not the extracted key array!) is passed in case the consistent method needs to consult it. On success, *recheck should be set to TRUE if the heap tuple needs to be rechecked against the query operator, or FALSE if the index test is exact. Optionally, an operator class for GIN can supply a fifth method: int comparePartial(Datum partial_key, Datum key, StrategyNumber n) Compare a partial-match query to an index key. Returns an integer whose sign indicates the result: less than zero means the index key does not match the query, but the index scan should continue; zero means that the index key does match the query; greater than zero indicates that the index scan should stop because no more matches are possible. The strategy number n of the operator that generated the partial match query is provided, in case its semantics are needed to determine when to end the scan. To support partial match queries, an operator class must provide the comparePartial method, and its extractQuery method must set the pmatch parameter when a partial-match query is encountered. See for details. Internally, a GIN index contains a B-tree index constructed over keys, where each key is an element of the indexed value (a member of an array, for example) and where each tuple in a leaf page is either a pointer to a B-tree over heap pointers (PT, posting tree), or a list of heap pointers (PL, posting list) if the list is small enough. 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 the compare support method). The extractQuery 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 the pmatch flag true. The key range is then searched using the comparePartial method. comparePartial must return zero for an actual match, 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. During a partial-match scan, all itemPointers for matching keys are OR'ed into a TIDBitmap. The scan fails if the TIDBitmap becomes lossy. In this case an error message will be reported with advice to increase work_mem. Create vs insert In most cases, insertion into a GIN index is slow due to the likelihood of many keys being inserted for each value. So, for bulk insertions into a table it is advisable to drop the GIN index and recreate it after finishing bulk insertion. 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. 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 slightly from the specified limit, depending on the query and the quality of the system's random number generator. GIN doesn't support full index scans: because there are often many keys per value, each heap pointer would be returned many times, and there is no easy way to prevent this. When extractQuery returns zero keys, GIN will emit an error. Depending on the operator, a void query might match all, some, or none of the indexed values (for example, every array contains the empty array, but does not overlap the empty array), and GIN cannot determine the correct answer, nor produce a full-index-scan result if it could determine that that was correct. It is not an error for extractValue to return zero keys, but in this case the indexed value will be unrepresented in the index. This is another reason why full index scan is not useful — it would miss such rows. 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. Note however that failure could still occur if the intermediate TIDBitmap becomes lossy. The PostgreSQL source distribution includes GIN operator classes for tsvector and for one-dimensional arrays of all internal types. Prefix searching in tsvector is implemented using the GIN partial match feature. The following contrib modules also contain GIN operator classes: hstore Module for storing (key, value) pairs intarray Enhanced support for int4[] pg_trgm Text similarity using trigram matching