# 12.11. 功能限制

The current limitations ofPostgreSQL's text search features are:

* The length of each lexeme must be less than 2K bytes
* The length of a`tsvector`(lexemes + positions) must be less than 1 megabyte
* The number of lexemes must be less than 264
* Position values in`tsvector`must be greater than 0 and no more than 16,383
* The match distance in a`<N`>(FOLLOWED BY)`tsquery`operator cannot be more than 16,384
* No more than 256 positions per lexeme
* The number of nodes (lexemes + operators) in a`tsquery`must be less than 32,768

For comparison, thePostgreSQL8.1 documentation contained 10,441 unique words, a total of 335,420 words, and the most frequent word“postgresql”was mentioned 6,127 times in 655 documents.

Another example — thePostgreSQLmailing list archives contained 910,989 unique words with 57,491,343 lexemes in 461,020 messages.