I'm running postgres 8.0.7, and I've got a table of orders with about
100,000 entries. I want to just look at the new orders, right now 104 of
them.
EXPLAIN ANALYZE
SELECT
order_id
FROM
orders
WHERE
order_statuses_id = (SELECT id FROM order_statuses WHERE id_name
= 'new');
Seq Scan on orders o (cost=1.20..11395.51 rows=7029 width=8) (actual
time=286.038..287.662 rows=104 loops=1)
Filter: (order_statuses_id = $0)
InitPlan
-> Seq Scan on order_statuses (cost=0.00..1.20 rows=1 width=4)
(actual time=0.030..0.039 rows=1 loops=1)
Filter: ((id_name)::text = 'new'::text)
Total runtime: 288.102 ms
The dreaded sequential scan. I've got an index on order_statuses_id and
I've VACUUM ANALYZEd the table, but I'm currently clustered on the
primary key (order_id).
With enable_seqscan = off, I get:
-------------------------------------------------
Index Scan using orders_status_btree_idx on orders o
(cost=4.64..12457.14 rows=7031 width=8) (actual time=0.164..0.664
rows=104 loops=1)
Index Cond: (order_statuses_id = $0)
InitPlan
-> Index Scan using order_statuses_id_name_key on order_statuses
(cost=0.00..4.64 rows=1 width=4) (actual time=0.128..0.134 rows=1 loops=1)
Index Cond: ((id_name)::text = 'new'::text)
Total runtime: 1.108 ms
If I hard-code the 'new' status ID, I get:
-------------------------------------------------
EXPLAIN ANALYZE
SELECT
order_id
FROM
orders
WHERE
order_statuses_id = 1;
Index Scan using orders_status_btree_idx on orders o
(cost=0.00..4539.65 rows=1319 width=8) (actual time=0.132..1.883
rows=104 loops=1)
Index Cond: (order_statuses_id = 1)
Total runtime: 2.380 ms
Here is the pg_stats entry for orders.order_statuses_id:
schemaname | tablename | attname | null_frac | avg_width |
n_distinct | most_common_vals | most_common_freqs
| histogram_bounds | correlation
------------+-----------+-------------------+-------------+-----------+------------+------------------+--------------------------------------+-------------------------+-------------
public | orders | order_statuses_id | 0.000208333 | 4
| 14 | {8,24,10,25} | {0.385417,0.242083,0.230625,0.07875} |
{1,7,7,9,9,9,9,9,23,26} | 0.740117
This is with SET STATISTICS = 16 on the column, since that's how many
different values the column can currently take.
Now, here's the thing - if I cluster on the index on order_statuses_id,
the original query produces:
Index Scan using orders_status_btree_idx on orders o
(cost=1.20..978.94 rows=8203 width=8) (actual time=0.097..0.598 rows=104
loops=1)
Index Cond: (order_statuses_id = $0)
InitPlan
-> Seq Scan on order_statuses (cost=0.00..1.20 rows=1 width=4)
(actual time=0.056..0.065 rows=1 loops=1)
Filter: ((id_name)::text = 'new'::text)
Total runtime: 1.042 ms
Estimated cost went way down. The pg_stats entry becomes:
schemaname | tablename | attname | null_frac | avg_width |
n_distinct | most_common_vals | most_common_freqs
| histogram_bounds | correlation
------------+-----------+-------------------+-----------+-----------+------------+------------------+----------------------------------------+---------------------+-------------
public | orders | order_statuses_id | 0 | 4
| 12 | {8,24,10,25} | {0.386458,0.244167,0.238333,0.0720833}
| {1,7,7,9,9,9,22,26} | 1
I'm hesitant to cluster on the order_statuses_id index, because there
are a lot of other queries using this table, many of which join on
order_id. I also feel like I ought to be able to get the planner to do
an index scan without hard-coding the order_statuses_id value.
Questions:
* What can I do to reduce the estimated row count on the query?
* Why does clustering drive down the estimated cost for the index scan
so much? Does a change in correlation from .72 to 1 make that much of a
difference?
* Can I convince my query planner to index scan without clustering on
the order_statuses_id index, or setting enable_seqscan = off?
Potential note of interest: This is a very wide, monolithic table - no
less than 100 columns, with several check constraints, foreign key
constraints, and indexes, including three functional indexes.
Side question: Sometimes, when I VACUUM ANALYZE the table, the pg_stats
entry for order_statuses_id has almost all of the possible values in
most_common_vals, instead of just a handful. Example:
schemaname | tablename | attname | null_frac | avg_width |
n_distinct | most_common_vals
|
most_common_freqs
| histogram_bounds | correlation
------------+-----------+-------------------+-----------+-----------+------------+-----------------------------------+------------------------------------------------------------------------------------------------------------------------------+------------------+-------------
public | orders | order_statuses_id | 0 | 4
| 13 | {8,24,10,25,9,7,23,26,1,22,2,5,4} |
{0.393125,0.240208,0.226042,0.07875,0.0275,0.0145833,0.0110417,0.00291667,0.00229167,0.001875,0.000625,0.000625,0.000416667}
| | 1
This doesn't appear to influence whether the index scan is chosen, but I
am curious as to why this happens.
