Обсуждение: partition query using Seq Scan even when index is present
With postgresql-8.3.6, I have many partitions inheriting a table. SELECT min() on the parent performs a Seq Scan, but SELECT min() on a child uses the index. Is this another case where the planner is not aware enough to come up with the best plan? I tried creating an index on the parent table to no avail. Is there a way to formulate the query so that it uses the index? Here is the general flavor: create table calls (caller text, ts timestamptz); create table calls_partition_2009_08 (check (ts >= '2009-08-01' and ts < '2009-09-01')) inherits (calls); create index calls_partition_2009_08_ts on calls_partition_2009_08 (ts); insert into calls_partition_2009_08 (ts) select to_timestamp(unix_time) from generate_series(extract(epoch from '2009-08-01'::timestamptz)::int, extract(epoch from '2009-08-31 23:59'::timestamptz)::int, 60) as unix_time; analyze calls_partition_2009_08; explain select min(ts) from calls; QUERY PLAN ----------------------------------------------------------------------------------------------- Aggregate (cost=780.50..780.51 rows=1 width=8) -> Append (cost=0.00..666.00 rows=45800 width=8) -> Seq Scan on calls (cost=0.00..21.60 rows=1160 width=8) -> Seq Scan on calls_partition_2009_08 calls (cost=0.00..644.40 rows=44640 width=8) (4 rows) explain select min(ts) from calls_partition_2009_08; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------- Result (cost=0.03..0.04 rows=1 width=0) InitPlan -> Limit (cost=0.00..0.03 rows=1 width=8) -> Index Scan using calls_partition_2009_08_ts on calls_partition_2009_08 (cost=0.00..1366.85 rows=44640 width=8) Filter: (ts IS NOT NULL) (5 rows)
Yep.... I ran into the exact same problem.
My solution was to create a pl/pgsql function to query the child tables: ( http://archives.postgresql.org/pgsql-performance/2008-11/msg00284.php)
If you find a better solution please share.
-Greg Jaman
On Wed, Sep 2, 2009 at 1:15 PM, Kenneth Cox <kenstir@gmail.com> wrote:
With postgresql-8.3.6, I have many partitions inheriting a table. SELECT min() on the parent performs a Seq Scan, but SELECT min() on a child uses the index. Is this another case where the planner is not aware enough to come up with the best plan? I tried creating an index on the parent table to no avail. Is there a way to formulate the query so that it uses the index? Here is the general flavor:
create table calls (caller text, ts timestamptz);
create table calls_partition_2009_08 (check (ts >= '2009-08-01' and ts < '2009-09-01')) inherits (calls);
create index calls_partition_2009_08_ts on calls_partition_2009_08 (ts);
insert into calls_partition_2009_08 (ts)
select to_timestamp(unix_time)
from generate_series(extract(epoch from '2009-08-01'::timestamptz)::int,
extract(epoch from '2009-08-31 23:59'::timestamptz)::int, 60) as unix_time;
analyze calls_partition_2009_08;
explain select min(ts) from calls;
QUERY PLAN
-----------------------------------------------------------------------------------------------
Aggregate (cost=780.50..780.51 rows=1 width=8)
-> Append (cost=0.00..666.00 rows=45800 width=8)
-> Seq Scan on calls (cost=0.00..21.60 rows=1160 width=8)
-> Seq Scan on calls_partition_2009_08 calls (cost=0.00..644.40 rows=44640 width=8)
(4 rows)
explain select min(ts) from calls_partition_2009_08;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Result (cost=0.03..0.04 rows=1 width=0)
InitPlan
-> Limit (cost=0.00..0.03 rows=1 width=8)
-> Index Scan using calls_partition_2009_08_ts on calls_partition_2009_08 (cost=0.00..1366.85 rows=44640 width=8)
Filter: (ts IS NOT NULL)
(5 rows)
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On Wed, Sep 2, 2009 at 4:15 PM, Kenneth Cox<kenstir@gmail.com> wrote: > With postgresql-8.3.6, I have many partitions inheriting a table. SELECT > min() on the parent performs a Seq Scan, but SELECT min() on a child uses > the index. Is this another case where the planner is not aware enough to > come up with the best plan? I tried creating an index on the parent table > to no avail. Is there a way to formulate the query so that it uses the > index? Here is the general flavor: > > create table calls (caller text, ts timestamptz); > create table calls_partition_2009_08 (check (ts >= '2009-08-01' and ts < > '2009-09-01')) inherits (calls); > create index calls_partition_2009_08_ts on calls_partition_2009_08 (ts); > insert into calls_partition_2009_08 (ts) > select to_timestamp(unix_time) > from generate_series(extract(epoch from '2009-08-01'::timestamptz)::int, > extract(epoch from '2009-08-31 > 23:59'::timestamptz)::int, 60) as unix_time; > analyze calls_partition_2009_08; > explain select min(ts) from calls; ATM, constraint exclusion mainly only supports queries of the form: SELECT ... WHERE 'x', with x being an expression in the check constraint. Table partitioning unfortunately is not a free lunch, you have to be aware of it at all times when writing queries vs your partitioned tables. merlin
Thank you, Greg! I tweaked your function to use recursion to search all inherited tables; my inheritance structure is two levels deep. This function is for integers only; I will copy/waste to create one for timestamps. Extra credit for anyone who can rewrite it to be polymorphic. -- Same as max(_colname) from _relname but much faster for inherited -- tables with an index on _colname. In postgresql-8.3.6 a naive query -- on a parent table will not use the indexes on the child tables. create or replace function partition_max_int(_relname text, _colname text) returns int AS $$ declare childtable RECORD; childres RECORD; maxval int; tmpval int; sql text; begin -- find max in this table (only) sql := 'select max('||_colname||') from only '||quote_ident(_relname); execute sql into maxval; -- recurse to find max in descendants FOR childtable in select pc.relname as relname from pg_class pc join pg_inherits pi on pc.oid=pi.inhrelid where inhparent=(select oid from pg_class where relname=_relname) LOOP tmpval := partition_max_int(childtable.relname, _colname); IF tmpval is not NULL and (tmpval > maxval or maxval is null) THEN maxval := tmpval; END IF; END LOOP; return maxval; end; $$ language 'plpgsql' STABLE;