Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL

From: Tom Lane
Subject: Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL
Date: ,
Msg-id: 9871.1115733198@sss.pgh.pa.us
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In response to: Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Greg Stark)
Responses: Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Greg Stark)
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"Hash index" vs. "b-tree index" (PostgreSQL 8.0)  (Ying Lu, )
 Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL 8.0)  (Neil Conway, )
  Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL 8.0)  (Christopher Petrilli, )
   Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Neil Conway, )
    Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  ("Jim C. Nasby", )
     Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Neil Conway, )
      Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  ("Jim C. Nasby", )
       Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Neil Conway, )
        Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Tom Lane, )
         Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Neil Conway, )
          Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Tom Lane, )
           Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Neil Conway, )
           Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Greg Stark, )
            Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Tom Lane, )
             Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Greg Stark, )
              Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Tom Lane, )
               Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Greg Stark, )
                Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Tom Lane, )
                 Federated PG servers -- Was: Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Mischa Sandberg, )
         Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  ("Jim C. Nasby", )
          Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Tom Lane, )
           Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  ("Jim C. Nasby", )
            Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Mischa Sandberg, )
        Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  ("Jim C. Nasby", )
         Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Tom Lane, )
         Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Mischa Sandberg, )
          Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Mark Lewis, )
          Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Tom Lane, )
           Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Mischa Sandberg, )
       Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Bruce Momjian, )
 Re: "Hash index" vs. "b-tree index" (PostgreSQL 8.0)  (David Roussel, )
 Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Tom Lane, )
  Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Neil Conway, )
 Re: [GENERAL] "Hash index" vs. "b-tree index" (PostgreSQL  (Greg Stark, )

Greg Stark <> writes:
> Tom Lane <> writes:
>> I think that efficient implementation of this would require explicitly
>> storing the hash code for each index entry,

> It seems that means doubling the size of the hash index. That's a pretty big
> i/o to cpu tradeoff.

Hardly.  The minimum possible size of a hash entry today is 8 bytes
header plus 4 bytes datum, plus there's a 4-byte line pointer to factor
in.  So under the most pessimistic assumptions, storing the hash code
would add 25% to the size.  (On MAXALIGN=8 hardware, it might cost you
nothing at all.)

> What if the hash index stored *only* the hash code? That could be useful for
> indexing large datatypes that would otherwise create large indexes.

Hmm, that could be a thought.

            regards, tom lane


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