Re: indexes problem

On 1/27/2010 7:32 AM, J Scanf wrote:
> Hi all. I have a problem with two tables of same structure:
> 'orders_stat_pre' and 'orders_stat_pre_new'.
> store=# \d orders_stat_pre
> Column | Type | Modifiers
> ----------------+----------------------------+-----------
> id | integer | not null
> user_name | text |
> category_name | character varying(10) |
> ctime | timestamp without timezone |
> Indexes:
> "orders_stat_pre_pkey" PRIMARY KEY, btree (id)
> "orders_stat_pre_user_idx" btree (user_name, category_name, ctime DESC)
>
>
> store=# \d orders_stat_pre_new
> Column | Type | Modifiers
> ----------------+----------------------------+-----------
> id | integer |
> user_name | text |
> category_name | character varying(10) |
> ctime | timestamp without timezone |
> Indexes:
> "orders_stat_pre_new_user_idx" btree (user_name, category_name, ctime DESC)
>
> I try to select last 10 orders from old table (i.e. 'orders_stat_pre'):
>
> store=# explain select * from orders_stat_pre where user_name = 'Alex'
> and category_name = 'Books' order by ctime desc limit 10;
> QUERY PLAN
> ---------------------------------------------------------------------------------------------------------------
> Limit (cost=0.00..40.40 rows=10 width=335)
> -> Index Scan using orders_stat_pre_user_idx on orders_stat_pre
> (cost=0.00..15505.87 rows=3838 width=335)
> Index Cond: ((user_name = 'Alex'::text) AND
> ((category_name)::text = 'Books'::text))
> (3 rows)
>
> Then I do the same query on new table (i.e. 'orders_stat_pre_new'):
>
> store=# explain select * from orders_stat_pre_new where user_name =
> 'Alex' and category_name = 'Books' order by ctime desc limit 10;
> QUERY PLAN
> ------------------------------------------------------------------------------------------------
> Limit (cost=1719969.83..1719969.86 rows=10 width=563)
> -> Sort (cost=1719969.83..1719981.08 rows=4499 width=563)
> Sort Key: ctime
> -> Seq Scan on orders_stat_pre_new (cost=0.00..1719872.61
> rows=4499 width=563)
> Filter: ((user_name = 'Alex'::text) AND
> ((category_name)::text = 'Books'::text))
> (5 rows)
>
> I'm confused on how can I optimize the last query? Or where I can find
> corresponding info. Thank you!
>

1) an "explain analyze" might give more useful info.
2) are your stats up do date? run 'analyze orders_stat_pre_new' and try
again
3) you might be indexing too much. An index on just user_name might be
enough. The extra fields (category_name and ctime) may not help you as
much as you think it will. A more complicated index (multiple fields)
makes it harder for PG to use. Drop that index and create one on just
user_name and compare the times.

-Andy