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Current implementation of FTS has some limitations.
Length of lexeme < 2K
Length of tsvector (lexemes + positions) < 1Mb
The number of lexemes < 432
0< Positional information < 16383
No more than 256 positions per lexeme
The number of nodes ( lexemes + operations) in tsquery < 32768
For comparison, PostgreSQL 8.1 documentation consists of 10441 unique words, total 335420 words and most frequent word 'postgresql' mentioned 6127 times in 655 documents.
Another example - PostgreSQL mailing list archive consists of 910989 unique words, total 57,491,343 lexemes in 461020 messages.