Re: partitioned table query question

On Dec 7, 2007, at 10:51 PM, Mason Hale wrote:

> I'm implementing table partitioning on 8.2.5 -- I've got the tables
> set up to partition based on the % 10 value of a key.
>
> My problem is that I can't get the planner to take advantage of the
> partitioning without also adding a key % 10 to the where clause.
> Is there any way around that?
>
> My child table definitions are:
>
> CREATE TABLE topic_version_page_0 (
> CHECK (topic_version_id % 10 = 0::integer )
> ) inherits (topic_version_page);
>
> ...
>
> CREATE TABLE topic_version_page_9 (
> CHECK (topic_version_id % 10 = 9::integer )
> ) inherits (topic_version_page);
>
>
> I've also created indexes and constraints for each child table, and
> an insert trigger on the master table (topic_version_page).
>
> If I include a 'topic_version_id % 10 = [some value]' in my query,
> then the partitioning shows up in the query plan:
>
> test=> explain select * from topic_version_page where
> topic_version_id % 10 = (102 % 10) and topic_version_id = 102;
> QUERY
> PLAN
> ----------------------------------------------------------------------
> ----------------------------------------------------------------
> Result (cost=4.27..19.23 rows=2 width=194)
> -> Append (cost= 4.27..19.23 rows=2 width=194)
> -> Bitmap Heap Scan on topic_version_page
> (cost=4.27..9.62 rows=1 width=194)
> Recheck Cond: (topic_version_id = 102)
> Filter: ((topic_version_id % 10) = 2)
> -> Bitmap Index Scan on
> index_topic_version_page_on_topic_version_id_and_created_at
> (cost=0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_2
> topic_version_page (cost=4.27..9.62 rows=1 width=194)
> Recheck Cond: (topic_version_id = 102)
> Filter: ((topic_version_id % 10) = 2)
> -> Bitmap Index Scan on
> index_topic_version_page_2_on_topic_version_id_and_page_id
> (cost=0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> (12 rows)
>
> But if I don't explicitly include a 'topic_version_id % 10' -- the
> plan gets much worse, checking every table (see below).
>
>
> test=> explain select * from topic_version_page where
> topic_version_id = 102;
> QUERY
> PLAN
> ----------------------------------------------------------------------
> ----------------------------------------------------------------
> Result (cost=4.27..105.68 rows=22 width=194)
> -> Append (cost= 4.27..105.68 rows=22 width=194)
> -> Bitmap Heap Scan on topic_version_page
> (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_on_topic_version_id_and_created_at (cost=
> 0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_0
> topic_version_page (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_0_on_topic_version_id_and_page_id
> (cost=0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_1
> topic_version_page (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_1_on_topic_version_id_and_page_id (cost=
> 0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_2
> topic_version_page (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_2_on_topic_version_id_and_page_id
> (cost=0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_3
> topic_version_page (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_3_on_topic_version_id_and_page_id (cost=
> 0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_4
> topic_version_page (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_4_on_topic_version_id_and_page_id
> (cost=0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_5
> topic_version_page (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_5_on_topic_version_id_and_page_id (cost=
> 0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_6
> topic_version_page (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_6_on_topic_version_id_and_page_id
> (cost=0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_7
> topic_version_page (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_7_on_topic_version_id_and_page_id (cost=
> 0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_8
> topic_version_page (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_8_on_topic_version_id_and_page_id
> (cost=0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> -> Bitmap Heap Scan on topic_version_page_9
> topic_version_page (cost=4.27..9.61 rows=2 width=194)
> Recheck Cond: (topic_version_id = 102)
> -> Bitmap Index Scan on
> index_topic_version_page_9_on_topic_version_id_and_page_id (cost=
> 0.00..4.27 rows=2 width=0)
> Index Cond: (topic_version_id = 102)
> (46 rows)
>
>
> Is there anyway to get the benefit of partitioning without adding a
> additional 'topic_version_id % 10' condition to every query that
> touches this table?
>
> Thanks in advance.

You beat me to the punch on this one. I was wanting to use modulo
operations for bin style partitioning as well, but this makes things
pretty awkward as well as unintuitive. So, to the postgres gurus:
What are the limitations of check constraints when used with
constraint exclusion? Is this really the intended behavior?

Erik Jones

Software Developer | Emma®
erik(at)myemma(dot)com
800.595.4401 or 615.292.5888
615.292.0777 (fax)

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