Re: [HACKERS] Searching for substring with tsearch(1/2)

[Teodor Sigaev]

I meant that the expansion of 'hu%' is done before and outside of
tsearch, so the question is how efficient will tsearch be for searching
for hudreds or thousands of words in one expression.
Ok, I see. The answer - bad. Index structure is signature tree with constant 
signature length, by default 2016 bits. Siganture makes by hashing word and sets 
bits number HASHVAL % 2016 to 1. So, if query has many terms and all terms are 
ored then there is a lot of signatures that matched by query. This means a lot 
of pages in index will be readed.


How hard (or sensible ;) would be creating such an index using GiST ?
As proved by tsearch GiST can cope well with many-to-many indexes.
Sorry, I don't understand. Do you mean that GiST supports one heap tuple in 
several index tuple? If yes then no :). GiST doesn't support this feature. I 
don't think that GiST may help in this situation.


but tsearch seems to support this, and tsearch uses GiST. Is this
functionality added entirely by tsearch ?
No, one heap tuple - one index tuple.

I'll try to explain index structure used by tsearch (three levels just for example):
Root page
 internal tuple 1  -> second level page 1
                       internal tuple 1.1 ->
                       internal tuple 1.2 ->
 internal tuple 2  -> second level page 2
                       internal tuple 2.1 ->
                       internal tuple 2.2 -> third level (leaf) page 2.2
                                              leaf tuple 2.2.1 -> heap tuple
                                              leaf tuple 2.2.2 -> heap tuple
leaf tuple contains one of two types of predicats:
  1 just lexemes (without psition information)
  2 if store size of first type is too big then tuple
    stores signature as described above.
internal tuple contains ored (super-imposed) signatures of childs.



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Teodor Sigaev                                  E-mail: [EMAIL PROTECTED]
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