Subject: Re: [HACKERS] Surjective functional indexes
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Stephen Frost <sfrost@snowman.net>,
 Michael Paquier <michael.paquier@gmail.com>,
 Simon Riggs <simon@2ndquadrant.com>, Christoph Berg <myon@debian.org>,
 Thomas Munro <thomas.munro@enterprisedb.com>,
 Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>
References: <20180322203743.reucnyd7pelkkxom@alvherre.pgsql>
From: Konstantin Knizhnik <k.knizhnik@postgrespro.ru>
Message-ID: <32bb7b6e-b138-9a7c-e74b-7af1365ec4ee@postgrespro.ru>
Date: Fri, 23 Mar 2018 18:39:24 +0300
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On 22.03.2018 23:37, Alvaro Herrera wrote:
> The rd_projidx (list of each nth element in the index list that is a
> projection index) thing looks odd.  Wouldn't it make more sense to have
> a list of index OIDs that are projection indexes?
>

rd_projidx is not a list, it is Bitmapset. It is just one of many bitmap 
sets in RelationData:

     Bitmapset  *rd_indexattr;    /* identifies columns used in indexes */
     Bitmapset  *rd_keyattr;        /* cols that can be ref'd by foreign 
keys */
     Bitmapset  *rd_pkattr;        /* cols included in primary key */
     Bitmapset  *rd_idattr;        /* included in replica identity index */

Yes, it is possible to construct Oid list of projectional indexes 
instead of having bitmap which marks some indexes in projectional, but I 
am not sure that
it will be more efficient both from CPU and memory footprint point of 
view (in most indexes bitmap will consists of just one integer). In any 
case, I do not think that it can have some measurable impact on 
performance or memory size:
number of indexes and especially projectional indexes will very rarely 
exceed 10.


-- 

Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company