On Tue, Aug 13, 2019 at 8:46 AM Thomas Kellerer <spam_ea...@gmx.net> wrote:
> I stumbled across this question on SO: > https://stackoverflow.com/questions/56517852 > > Disregarding the part about Postgres 9.3, the example for Postgres 11 > looks a bit confusing. > > There is a script to setup test data in that question: > > ==== start of script ==== > > create table foo ( > foo_id integer not null, > foo_name varchar(10), > constraint foo_pkey primary key (foo_id) > ); > > insert into foo > (foo_id, foo_name) > values > (1, 'eeny'), > (2, 'meeny'), > (3, 'miny'), > (4, 'moe'), > (5, 'tiger'), > (6, 'toe'); > > create table foo_bar_baz ( > foo_id integer not null, > bar_id integer not null, > baz integer not null, > constraint foo_bar_baz_pkey primary key (foo_id, bar_id, baz), > constraint foo_bar_baz_fkey1 foreign key (foo_id) > references foo (foo_id) > ) partition by range (foo_id) > ; > > create table if not exists foo_bar_baz_0 partition of foo_bar_baz for > values from (0) to (1); > create table if not exists foo_bar_baz_1 partition of foo_bar_baz for > values from (1) to (2); > create table if not exists foo_bar_baz_2 partition of foo_bar_baz for > values from (2) to (3); > create table if not exists foo_bar_baz_3 partition of foo_bar_baz for > values from (3) to (4); > create table if not exists foo_bar_baz_4 partition of foo_bar_baz for > values from (4) to (5); > create table if not exists foo_bar_baz_5 partition of foo_bar_baz for > values from (5) to (6); > > with foos_and_bars as ( > select ((random() * 4) + 1)::int as foo_id, bar_id::int > from generate_series(0, 1499) as t(bar_id) > ), bazzes as ( > select baz::int > from generate_series(1, 1500) as t(baz) > ) > insert into foo_bar_baz (foo_id, bar_id, baz) > select foo_id, bar_id, baz > from bazzes as bz > join foos_and_bars as fab on mod(bz.baz, fab.foo_id) = 0; > > ==== end of script ==== > > I see the some strange behaviour similar to to what is reported in the > comments to that question: > > When I run the test query immediately after populating the tables with the > sample data: > > explain analyze > select count(*) > from foo_bar_baz as fbb > join foo on fbb.foo_id = foo.foo_id > where foo.foo_name = 'eeny' > > I do see an "Index Only Scan .... (never executed)" in the plan for the > irrelevant partitions: > > https://explain.depesz.com/s/AqlE > > However once I run "analyze foo_bar_baz" (or "vacuum analyze"), Postgres > chooses to do a "Parallel Seq Scan" for each partition: > > https://explain.depesz.com/s/WwxE > > Why does updating the statistics mess up (runtime) partition pruning? > > > I played around with random_page_cost and that didn't change anything. > I tried to create extended statistics on "foo(id, name)" so that the > planner would no, that there is only one name per id. No change. > > I saw the above behaviour when running this on Windows 10 (my Laptop) or > CentOS 7 (a test environment on a VM) > > On the CentOS server default_statistics_target is set to 100, on my laptop > it is set to 1000 > > In both cases the Postgres version was 11.4 > > Any ideas? > > Thomas > > Ran into the same behaviour of the planner. The amount of rows in the partitions influence the statistics being generated and the statistics in turn influence the plan chosen. I managed to force the "correct" plan by manually setting the n_distinct statistics for the partitioned table. E.g.: alter table foo_bar_baz alter column foo_id set ( n_distinct=-1, n_distinct_inherited=-1); With a certain number of rows in the partitions the analyser sets the n_distinct value for the partitioned table to the number of unique partition keys and the n_distinct value for the individual partitions to number of unique partition keys in that partition. Unfortunately this causes the planner to pick a plan that doesn't allow for execution pruning, resulting in very slow execution times. Regards, Sverre
