From: Tom Lane <tgl@sss.pgh.pa.us>
To: Rhett Garber <rhettg@gmail.com>
Cc: Havasvölgyi Ottó <h.otto@freemail.hu>
Cc: pgsql-performance@postgresql.org
Subject: Re: Why hash join instead of nested loop?
Date: Sat, 06 Aug 2005 01:03:51 -0400
Message-ID: <15425.1123304631@sss.pgh.pa.us> (raw)
In-Reply-To: <41b0fe8905080516164464e04f@mail.gmail.com>
References: <41b0fe890508051135712a086b@mail.gmail.com>
<001801c59a0a$f4be7480$9a00a8c0@OTTO>
<41b0fe8905080516164464e04f@mail.gmail.com>
Rhett Garber <rhettg@gmail.com> writes:
> Hash Join (cost=5.96..7.04 rows=1 width=14) (actual
> time=10.591..10.609 rows=1 loops=1)
> Hash Cond: ("outer".id = "inner".obj2)
> -> Seq Scan on rtmessagestate (cost=0.00..1.05 rows=5 width=14)
> (actual time=0.011..0.022 rows=5 loops=1)
> -> Hash (cost=5.96..5.96 rows=1 width=4) (actual
> time=0.109..0.109 rows=0 loops=1)
> -> Index Scan using connection_regid_obj1_index on
> connection (cost=0.00..5.96 rows=1 width=4) (actual time=0.070..0.076
> rows=1 loops=1)
> Index Cond: ((connection_registry_id = 40105) AND (obj1
> = 73582)) Total runtime: 11.536 ms
> (7 rows)
[ scratches head... ] If the hash table build takes only 0.109 msec
and loads only one row into the hash table, and the scan of
rtmessagestate takes only 0.022 msec and produces only 5 rows, it is
real hard to see how the join takes 10.609 msec overall. Unless the id
and obj2 columns are of a datatype with an incredibly slow equality
function. What is the datatype involved here, anyway? And what PG
version are we speaking of?
regards, tom lane
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, rhettg@gmail.com, h.otto@freemail.hu
Subject: Re: Why hash join instead of nested loop?
In-Reply-To: <15425.1123304631@sss.pgh.pa.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox