Multitenancy optimization

Hi hacker,

Postgres is quite frequently used in different Internet services with
multi-tenant architecture.
It means that all object stored in the database have something like
"tenant_id" foreign key.
This key is used in all queries, i.e.

     select * from Product where tenant_id=? and product_name=?;

The problem is that columns "tenant_id" and "product_name" are
frequently highly correlated (for example if this product is produced
just by one company).
And Postgres knows nothing about this correlation and so makes incorrect
estimation of selectivity of this predicate.

Certainly it is possible to create multicolumn statistics to notify
Postgres about columns correlation.
But unfortunately it is not good and working solution.

First of all we have to create multicolumn statistic for all possible
combinations of table's attributes including "tenant_id".
It is very inconvenient and inefficient.

Second - right now multicolumn statistic is not used for calculating
join selectivity. And for joins estimation errors are most critical,
causing Postgres to choose bad execution plans.

From my point of view the best solution is to make Postgres take in
account possible statistics errors and choose "stable" plan which
cost is not significantly increased in case of estimation errors. But it
requires huge refactoring of optimizer.

Right now I have information that some of Postgres customer which faced
with such problem just hacked calc_joinrel_size_estimate function,
checking attribute name and if it is "tenant_id"  then do not take its
selectivity in account.
It leads to good query plans but certainly can not be considered as
acceptable solution.

I thought about more straightforward ways for reaching the same effect.
Right now Postgres allows to explicitly specify number of distinct
values for the attribute:

      alter table foo alter column x set (n_distinct=1);

Unfortunately just setting it to 1 doesn't work. Postgres calculates
selectivity based on MCV or histogram and not using n_distinct value.
It is also possible to disable collection of statistic for this columns:

      alter table foo alter column x set statistics 0;

But in this case Postgres is choosing DEFAULT_NUM_DISTINCT despite to
n_distinct option specified for this attribute.
I propose small patch which makes Postgres to use explicitly specified
n_distinct attribute option value when no statistic is available.

This test illustrating how it works (without this patch estimation for
this query is 1 row):

postgres=# create table foo(x integer, y integer);
CREATE TABLE
postgres=# insert into foo values (generate_series(1,100000)/10,
generate_series(1,100000)/10);
INSERT 0 100000
postgres=# alter table foo alter column x set (n_distinct=1);
ALTER TABLE
postgres=# alter table foo alter column x set statistics 0;
ALTER TABLE
postgres=# analyze foo;
ANALYZE
postgres=# explain select * from foo where x=100 and y=100;
                      QUERY PLAN
-------------------------------------------------------
 Seq Scan on foo  (cost=0.00..1943.00 rows=10 width=8)
   Filter: ((x = 100) AND (y = 100))
(2 rows)

Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company