# 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.


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# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.postgresql.tw/15/the-sql-language/12.-quan-wen-jian-suo/12.11.-gong-neng-xian-zhi.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.