12.9. GIN及GiST索引型別

There are two kinds of indexes that can be used to speed up full text searches. Note that indexes are not mandatory for full text searching, but in cases where a column is searched on a regular basis, an index is usually desirable.

CREATE INDEX

name

ON

table

USING GIN (

column

);

Creates a GIN (Generalized Inverted Index)-based index. The_column_must be oftsvectortype.

CREATE INDEX

name

ON

table

USING GIST (

column

);

Creates a GiST (Generalized Search Tree)-based index. The_column_can be oftsvectorortsquerytype.

GIN indexes are the preferred text search index type. As inverted indexes, they contain an index entry for each word (lexeme), with a compressed list of matching locations. Multi-word searches can find the first match, then use the index to remove rows that are lacking additional words. GIN indexes store only the words (lexemes) oftsvectorvalues, and not their weight labels. Thus a table row recheck is needed when using a query that involves weights.

A GiST index islossy, meaning that the index might produce false matches, and it is necessary to check the actual table row to eliminate such false matches. (PostgreSQLdoes this automatically when needed.) GiST indexes are lossy because each document is represented in the index by a fixed-length signature. The signature is generated by hashing each word into a single bit in an n-bit string, with all these bits OR-ed together to produce an n-bit document signature. When two words hash to the same bit position there will be a false match. If all words in the query have matches (real or false) then the table row must be retrieved to see if the match is correct.

Lossiness causes performance degradation due to unnecessary fetches of table records that turn out to be false matches. Since random access to table records is slow, this limits the usefulness of GiST indexes. The likelihood of false matches depends on several factors, in particular the number of unique words, so using dictionaries to reduce this number is recommended.

Note thatGINindex build time can often be improved by increasingmaintenance_work_mem, whileGiSTindex build time is not sensitive to that parameter.

Partitioning of big collections and the proper use of GIN and GiST indexes allows the implementation of very fast searches with online update. Partitioning can be done at the database level using table inheritance, or by distributing documents over servers and collecting search results using thedblinkmodule. The latter is possible because ranking functions use only local information.