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From: aoki@CS.Berkeley.EDU (Paul M. Aoki)
To: Wojtek Bogusz <Wojtek.Bogusz@fuw.edu.pl>
Cc: Postgres Mailing List <postgres@postgres.Berkeley.EDU>
Subject: Re: New type and pg_operator. 
Reply-To: aoki@CS.Berkeley.EDU (Paul M. Aoki)
In-reply-to: Your message of Fri, 15 Jul 1994 14:00:40 +0200 (MET DST) 
	     <Pine.3.89.9407151351.C21375-0100000@ccfs1> 
Date: Fri, 15 Jul 94 05:34:41 -0700
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the info needed to add new operators and stuff is in chapter 13 of
the user manual.

Wojtek Bogusz <Wojtek.Bogusz@fuw.edu.pl> writes:
> surprising for me that for all operators connected with char/text types
> field oprprec (operator precedence) is equal 0. Why ?

doesn't work anyway.

>  Does exist some (simple :-) explanation what join strategies mean ?

sure, in any database textbook..

the nickle version:

nestloop join - compare everything in A to everything in B

sortmerge join - exploit sorted order to avoid scanning things in 
B known not to match the current element of A (too big, too small,
whatever).

hash join - join only those elements that fall into the same hash 
bucket (same idea - if it doesn't hash into the same bucket, it's 
not bitwise equal, so you don't have to look at it)

> And why hash join strategy is defined (oprcanhash = t) just for "=" ?

unless you use arcane order-preserving hash functions, hash join 
only makes sense for bitwise equality.

> And why
> merge-sort join strategy is not defined for all char/text operators ie. 
> oprrsortop and oprlsort are equal to 0 ?

i have no idea why not.
--
  Paul M. Aoki          |  University of California at Berkeley
  aoki@CS.Berkeley.EDU  |  Dept. of EECS, Computer Science Division (#1776) 
                        |  Berkeley, CA 94720-1776

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