$PostgreSQL: pgsql/src/backend/access/gin/README,v 1.6 2008/07/08 03:25:42 neilc Exp $ Gin for PostgreSQL ================== Gin was sponsored by jfg://networks (http://www.jfg-networks.com/) Gin stands for Generalized Inverted Index and should be considered as a genie, not a drink. Generalized means that the index does not know which operation it accelerates. It instead works with custom strategies, defined for specific data types (read "Index Method Strategies" in the PostgreSQL documentation). In that sense, Gin is similar to GiST and differs from btree indices, which have predefined, comparison-based operations. An inverted index is an index structure storing a set of (key, posting list) pairs, where 'posting list' is a set of documents in which the key occurs. (A text document would usually contain many keys.) The primary goal of Gin indices is support for highly scalable, full-text search in PostgreSQL. Gin consists of a B-tree index constructed over entries (ET, entries tree), where each entry is an element of the indexed value (element of array, lexeme for tsvector) and where each tuple in a leaf page is either a pointer to a B-tree over item pointers (PT, posting tree), or a list of item pointers (PL, posting list) if the tuple is small enough. Note: There is no delete operation for ET. The reason for this is that in our experience, the set of distinct words in a large corpus changes very rarely. This greatly simplifies the code and concurrency algorithms. Gin comes with built-in support for one-dimensional arrays (eg. integer[], text[]), but no support for NULL elements. The following operations are available: * contains: value_array @> query_array * overlaps: value_array && query_array * is contained by: value_array <@ query_array Synopsis -------- =# create index txt_idx on aa using gin(a); Features -------- * Concurrency * Write-Ahead Logging (WAL). (Recoverability from crashes.) * User-defined opclasses. (The scheme is similar to GiST.) * Optimized index creation (Makes use of maintenance_work_mem to accumulate postings in memory.) * Text search support via an opclass * Soft upper limit on the returned results set using a GUC variable: gin_fuzzy_search_limit Gin Fuzzy Limit --------------- There are often situations when a full-text search returns a very large set of results. Since reading tuples from the disk and sorting them could take a lot of time, this is unacceptable for production. (Note that the search itself is very fast.) Such queries usually contain very frequent lexemes, so the results are not very helpful. To facilitate execution of such queries Gin has a configurable soft upper limit on the size of the returned set, determined by the 'gin_fuzzy_search_limit' GUC variable. This is set to 0 by default (no limit). If a non-zero search 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 slightly differ from the specified limit, depending on the query and the quality of the system's random number generator. From experience, a value of 'gin_fuzzy_search_limit' in the thousands (eg. 5000-20000) works well. This means that 'gin_fuzzy_search_limit' will have no effect for queries returning a result set with less tuples than this number. Concurrency ----------- The entry tree and each posting tree is a B-tree, with right-links connecting sibling pages at the same level. This is the same structure that is used in the regular B-tree indexam (invented by Lehman & Yao), but we don't support scanning a GIN trees backwards, so we don't need left-links. To avoid deadlocks, B-tree pages must always be locked in the same order: left to right, and bottom to top. When searching, the tree is traversed from top to bottom, so the lock on the parent page must be released before descending to the next level. Concurrent page splits move the keyspace to right, so after following a downlink, the page actually containing the key we're looking for might be somewhere to the right of the page we landed on. In that case, we follow the right-links until we find the page we're looking for. To delete a page, the page's left sibling, the target page, and its parent, are locked in that order, and the page is marked as deleted. However, a concurrent search might already have read a pointer to the page, and might be just about to follow it. A page can be reached via the right-link of its left sibling, or via its downlink in the parent. To prevent a backend from reaching a deleted page via a right-link, when following a right-link the lock on the previous page is not released until the lock on next page has been acquired. The downlink is more tricky. A search descending the tree must release the lock on the parent page before locking the child, or it could deadlock with a concurrent split of the child page; a page split locks the parent, while already holding a lock on the child page. However, posting trees are only fully searched from left to right, starting from the leftmost leaf. (The tree-structure is only needed by insertions, to quickly find the correct insert location). So as long as we don't delete the leftmost page on each level, a search can never follow a downlink to page that's about to be deleted. The previous paragraph's reasoning only applies to searches, and only to posting trees. To protect from inserters following a downlink to a deleted page, vacuum simply locks out all concurrent insertions to the posting tree, by holding a super-exclusive lock on the posting tree root. Inserters hold a pin on the root page, but searches do not, so while new searches cannot begin while root page is locked, any already-in-progress scans can continue concurrently with vacuum. In the entry tree, we never delete pages. (This is quite different from the mechanism the btree indexam uses to make page-deletions safe; it stamps the deleted pages with an XID and keeps the deleted pages around with the right-link intact until all concurrent scans have finished.) Limitations ----------- * No support for multicolumn indices * Gin doesn't uses scan->kill_prior_tuple & scan->ignore_killed_tuples * Gin searches entries only by equality matching. This may be improved in future. * Gin doesn't support full scans of indices. * Gin doesn't index NULL values. Open Items ---------- We appreciate any comments, help and suggestions. * Teach optimizer/executor that GIN is intrinsically clustered. i.e., it always returns ItemPointer in ascending order. * Tweak gincostestimate. * GIN stores several ItemPointer to heap tuple, so VACUUM FULL produces this warning message: WARNING: index "idx" contains 88395 row versions, but table contains 51812 row versions HINT: Rebuild the index with REINDEX. **** Workaround added TODO ---- Nearest future: * Opclasses for all types (no programming, just many catalog changes). Distant future: * Replace B-tree of entries to something like GiST * Add multicolumn support * Optimize insert operations (background index insertion) Authors ------- All work was done by Teodor Sigaev (teodor@sigaev.ru) and Oleg Bartunov (oleg@sai.msu.su).