Re: Why LIMIT after scanning the table?

If you only what to know if there is more than 100 rows, why not do:

if exists (
SELECT 1 FROM email_rank WHERE project_id = :ProjectID OFFSET 100 LIMIT
1
)


"Jim C. Nasby" wrote:
> 
> 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
> cute...
> 
> explain select count(*)
>     FROM () 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?
> 
> FYI...
> 
>               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@nasby.net
> Member: Triangle Fraternity, Sports Car Club of America
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