Re: [PERFORM] Performance of information_schema with many schemataand tables - Mailing list pgsql-performance

On Wednesday 28 June 2017 02:00 PM, Ulf Lohbrügge wrote:
> Nope, I didn't try that yet. But I don't have the impression that reindexing the indexes in information_schema will help. The table information_schema.tables consists of the following indexes:
>
>     "pg_class_oid_index" UNIQUE, btree (oid)
>     "pg_class_relname_nsp_index" UNIQUE, btree (relname, relnamespace)
>     "pg_class_tblspc_relfilenode_index" btree (reltablespace, relfilenode)

information_schema.tables is not a table, it's a view; at least on 9.5[0]. These indexes you list are actually indexes on the pg_catalog.pg_class table.

>
> The costly sequence scan in question on pg_class happens with the following WHERE clause:
>
> WHERE (c.relkind = ANY (ARRAY['r'::"char", 'v'::"char", 'f'::"char"])) AND NOT pg_is_other_temp_schema(nc.oid) AND (pg_has_role(c.relowner, 'USAGE'::text) OR has_table_privilege(c.oid, 'SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, TRIGGER'::text) OR has_any_column_privilege(c.oid, 'SELECT, INSERT, UPDATE, REFERENCES'::text));

This is not the bottleneck WHERE clause the query plan from your first mail shows. That one is:

((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)))

I can say with certainty that an index on pg_catalog.pg_class.relname is going to speed this up. Postgres doesn't allow modifying system catalogs, but the `REINDEX SYSTEM <dbname>;` command should rebuild the system indexes and pg_catalog.pg_class.relname should be included in them (I tested on 9.6).

Do try that once. If you still see sequential scans, check what indexes are present on pg_catalog.pg_class.

>
> Besides pg_class_oid_index none of the referenced columns is indexed. I tried to add an index on relowner but didn't succeed because the column is used in the function call pg_has_role and the query is still forced to do a sequence scan.
>
> Regards,
> Ulf
>
> 2017-06-28 3:31 GMT+02:00 Pritam Baral <pritam@pritambaral.com <mailto:pritam@pritambaral.com>>:

>
>     On Wednesday 28 June 2017 05:27 AM, Ulf Lohbrügge wrote:
>     > 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!
>
>     Have you tried a `REINDEX SYSTEM <dbname>`?
>
>     >
>     > Regards,
>     > Ulf
>
>     --
>     #!/usr/bin/env regards
>     Chhatoi Pritam Baral
>
>
>
>     --

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[0]: https://www.postgresql.org/docs/9.5/static/infoschema-tables.html

--
#!/usr/bin/env regards
Chhatoi Pritam Baral