Re: why am I getting a seq scan on this query?

Mark Harrison wrote:
> I'm expecting this to do an indexed scan... any clue why it's not? This is
> with PG 7.4.

Someone might have a better idea but my guess is that PG things the
seq_scan would be faster. You could try decreasing your
random_page_cost. I have also heard that setting your (although I
haven't tested this) effective_cache_size higher then normal helps in
these scenarios but your mileage may vary.

Sincerely,

Joshua D. Drake

>
> Thanks!!
>
> planb=# explain select id,shotname from df_files where
> showid=30014515::bigint;
> QUERY PLAN
> -------------------------------------------------------------------
> Seq Scan on df_files (cost=0.00..791035.45 rows=540370 width=22)
> Filter: (showid = 30014515::bigint)
> (2 rows)
>
>
>
> planb=# \d df_files;
> Table "public.df_files"
> Column | Type | Modifiers
> -----------+-----------------------------+-----------
> id | bigint | not null
> showid | bigint | not null
> shotname | character varying(256) | not null
> elemname | character varying(256) | not null
> frameno | character varying(12) | not null
> ext | character varying(12) | not null
> filename | character varying(256) | not null
> filesize | bigint |
> locked | boolean |
> timestamp | timestamp without time zone |
> Indexes:
> "df_files_pkey" primary key, btree (id)
> "df_files_elemname" btree (elemname)
> "df_files_ext" btree (ext)
> "df_files_filename" btree (filename)
> "df_files_frameno" btree (frameno)
> "df_files_shotname" btree (shotname)
> "df_files_show" btree (showid)
> "df_files_showid" btree (showid)
>
> planb=# select count(*) from df_files where showid=30014515::bigint;
> count
> --------
> 528362
> (1 row)
>
> Time: 420598.071 ms
> planb=# select count(*) from df_files;
> count
> ----------
> 24415513
> (1 row)
>
> Time: 306554.085 ms
>
> ---------------------------(end of broadcast)---------------------------
> TIP 6: explain analyze is your friend

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