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 atsvector(lexemes + positions) must be less than 1 megabyte
• The number of lexemes must be less than 264
• Position values intsvectormust be greater than 0 and no more than 16,383
• The match distance in a<N>(FOLLOWED BY)tsqueryoperator cannot be more than 16,384
• No more than 256 positions per lexeme
• The number of nodes (lexemes + operators) in atsquerymust 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.