Performance of information_schema with many schemata and tables

Hi all,

we use schemata to separate our customers in a multi-tenant setup (9.5.7,
Debian stable). Each tenant is managed in his own schema with all the
tables that only he can access. All tables in all schemata are the same in
terms of their DDL: Every tenant uses e.g. his own table 'address'. We
currently manage around 1200 schemata (i.e. tenants) on one cluster. Every
schema consists currently of ~200 tables - so we end up with ~240000 tables
plus constraints, indexes, sequences et al.

Our current approach is quite nice in terms of data privacy because every
tenant is isolated from all other tenants. A tenant uses his own user that
gives him only access to the corresponding schema. Performance is great for
us - we didn't expect Postgres to scale so well!

But performance is pretty bad when we query things in the
information_schema:

SELECT
*
FROM information_schema.tables
WHERE table_schema = 'foo'
AND table_name = 'bar';``

Above query results in a large sequence scan with a filter that removes
1305161 rows:

QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Nested Loop Left Join (cost=0.70..101170.18 rows=3 width=265) (actual
time=383.505..383.505 rows=0 loops=1)
-> Nested Loop (cost=0.00..101144.65 rows=3 width=141) (actual
time=383.504..383.504 rows=0 loops=1)
Join Filter: (nc.oid = c.relnamespace)
-> Seq Scan on pg_class c (cost=0.00..101023.01 rows=867
width=77) (actual time=383.502..383.502 rows=0 loops=1)
Filter: ((relkind = ANY ('{r,v,f}'::"char"[])) AND
(((relname)::information_schema.sql_identifier)::text = 'bar'::text) AND
(pg_has_role(relowner, 'USAGE'::text) OR has_table_privilege(oid, 'SELECT,
INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, TRIGGER'::text) OR
has_any_column_privilege(oid, 'SELECT, INSERT, UPDATE, REFERENCES'::text)))
Rows Removed by Filter: 1305161
-> Materialize (cost=0.00..56.62 rows=5 width=68) (never
executed)
-> Seq Scan on pg_namespace nc (cost=0.00..56.60 rows=5
width=68) (never executed)
Filter: ((NOT pg_is_other_temp_schema(oid)) AND
(((nspname)::information_schema.sql_identifier)::text = 'foo'::text))
-> Nested Loop (cost=0.70..8.43 rows=1 width=132) (never executed)
-> Index Scan using pg_type_oid_index on pg_type t
(cost=0.42..8.12 rows=1 width=72) (never executed)
Index Cond: (c.reloftype = oid)
-> Index Scan using pg_namespace_oid_index on pg_namespace nt
(cost=0.28..0.30 rows=1 width=68) (never executed)
Index Cond: (oid = t.typnamespace)
Planning time: 0.624 ms
Execution time: 383.784 ms
(16 rows)

We noticed the degraded performance first when using the psql cli. Pressing
tab after beginning a WHERE clause results in a query against the
information_schema which is pretty slow and ends in "lag" when trying to
enter queries.

We also use Flyway (https://flywaydb.org/) to handle our database
migrations. Unfortunately Flyway is querying the information_schema to
check if specific tables exist (I guess this is one of the reasons
information_schema exists) and therefore vastly slows down the migration of
our tenants. Our last migration run on all tenants (schemata) almost took
2h because the above query is executed multiple times per tenant. The
migration run consisted of multiple sql files to be executed and triggered
more than 10 queries on information_schema per tenant.

I don't think that Flyway is to blame because querying the
information_schema should be a fast operation (and was fast for us when we
had less schemata). I tried to speedup querying pg_class by adding indexes
(after enabling allow_system_table_mods) but didn't succeed. The function
call 'pg_has_role' is probably not easy to optimize.

Postgres is really doing a great job to handle those many schemata and
tables but doesn't scale well when querying information_schema. I actually
don't want to change my current multi-tenant setup (one schema per tenant)
as it is working great but the slow information_schema is killing our
deployments.

Are there any other options besides switching from
one-schema-per-tenant-approach? Any help is greatly appreciated!

Regards,
Ulf