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Why LIMIT after scanning the table?

From: "Jim C(dot) Nasby" <jim(at)nasby(dot)net>
To: pgsql-performance(at)postgresql(dot)org
Subject: Why LIMIT after scanning the table?
Date: 2003-04-30 06:04:25
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
I'm doing something where I just need to know if we have more than 100
rows in a table. Not wanting to scan the whole table, I thought I'd get

explain select count(*)
    FROM (SELECT * FROM email_rank WHERE project_id = :ProjectID LIMIT 100) AS t1;
                                     QUERY PLAN                                      
 Aggregate  (cost=111.32..111.32 rows=1 width=48)
   ->  Subquery Scan t1  (cost=0.00..111.07 rows=100 width=48)
         ->  Limit  (cost=0.00..111.07 rows=100 width=48)
               ->  Seq Scan on email_rank  (cost=0.00..76017.40 rows=68439 width=48)
                     Filter: (project_id = 24)

The idea is that the inner-most query would only read the first 100 rows
it finds, then stop. Instead, if explain is to be believed (and speed
testing seems to indicate it's accurate), we'll read the entire table,
*then* pick the first 100 rows. Why is that?


              Table "public.email_rank"
        Column         |  Type   |     Modifiers      
 project_id            | integer | not null
 id                    | integer | not null
 first_date            | date    | not null
 last_date             | date    | not null
 day_rank              | integer | not null default 0
 day_rank_previous     | integer | not null default 0
 overall_rank          | integer | not null default 0
 overall_rank_previous | integer | not null default 0
 work_today            | bigint  | not null default 0
 work_total            | bigint  | not null default 0
Indexes: email_rank_pkey primary key btree (project_id, id),
         email_rank__day_rank btree (project_id, day_rank),
         email_rank__overall_rank btree (project_id, overall_rank)

Jim C. Nasby (aka Decibel!)                    jim(at)nasby(dot)net
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Subject: Re: Is 292 inserts/sec acceptable performance ?
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