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Re: Thanks, naming conventions, and count()

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Casey Lyon <casey(at)earthcars(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Thanks, naming conventions, and count()
Date: 2001-04-30 03:43:18
Message-ID: 200104300343.f3U3hIB12175@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
> It certainly works quickly for smaller tables, however the 21.7 million
> record table I ran this on takes a touch longer as shown here:
> 
> database=# explain select count(*) from table;
> NOTICE:  QUERY PLAN:
> 
> Aggregate  (cost=478056.20..478056.20 rows=1 width=0)
>    ->  Seq Scan on table  (cost=0.00..423737.76 rows=21727376 width=0)
> 
> EXPLAIN
> 
> However I noted explain provides rows as part of it's data; from what
> I've seen this loses precision over time or with large data imports,
> though; at least until the table is vacuumed again.

I guess I was saying that an index scan could take longer because it has
to walk the btree.  However it only has one column of the table, so it
may be faster.  I never measured the two, but the heap access needed for
the index scan currently is a performance killer.  Sequential is faster
than all those random heap lookups from the index.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

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