Postgresql – List foreign keys dependencies that refer to a given table

information-schemapostgresql

RDBMS: PostgreSQL 9.6

I desire to list the foreign keys for a given table and schema. I am about 90 % there but I also desire to know what options were used to make the fkey. Example on delete restrict or on delete update

create or replace function it.table_foreign_keys_getlist(_schema text
                                                       , _table text) 
                           returns TABLE(primary_key_ns text
                                       , primary_key_table text
                                       , foreign_key_ns text
                                       , foreign_key_table text
                                        )
     language sql
 as $$
 SELECT
     n1.nspname::TEXT  AS primary_key_ns,
     c1.relname::TEXT  AS primary_key_table,
     n2.nspname::TEXT  AS foreign_key_ns,
     c2.relname::TEXT  AS foreign_key_table
 FROM pg_catalog.pg_constraint c
 JOIN ONLY pg_catalog.pg_class c1     ON c1.oid = c.confrelid
 JOIN ONLY pg_catalog.pg_class c2     ON c2.oid = c.conrelid
 JOIN ONLY pg_catalog.pg_namespace n1 ON n1.oid = c1.relnamespace
 JOIN ONLY pg_catalog.pg_namespace n2 ON n2.oid = c2.relnamespace
 WHERE c1.relkind = 'r'
   AND c.contype  = 'f'
   AND CASE WHEN _schema = '' THEN TRUE ELSE n1.nspname = _schema END
   AND CASE WHEN _table  = '' THEN TRUE ELSE c1.relname = _table  END
 ORDER BY 1,2,3,4;
 $$;

Best Answer

You are looking for the columns confupdtype and confdeltype in the catalog pg_constraint.

The codes mean:

Foreign key deletion action code: a = no action, r = restrict, c = cascade, n = set null, d = set default

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You write:

Each customer can have multiple sites, but only one should be displayed in this list.

Yet, your query retrieves all rows. That would be a point to optimize. But you also do not define which site is to be picked.

Either way, it does not matter much here. Your EXPLAIN shows only 5026 rows for the site scan (5018 for the customer scan). So hardly any customer actually has more than one site. Did you ANALYZE your tables before running EXPLAIN?

From the numbers I see in your EXPLAIN, indexes will give you nothing for this query. Sequential table scans will be the fastest possible way. Half a second is rather slow for 5000 rows, though. Maybe your database needs some general performance tuning?

Maybe the query itself is faster, but "half a second" includes network transfer? EXPLAIN ANALYZE would tell us more.

If this query is your bottleneck, I would suggest you implement a materialized view.

After you provided more information I find that my diagnosis pretty much holds.

The query itself needs 27 ms. Not much of a problem there. "Half a second" was the kind of misunderstanding I had suspected. The slow part is the network transfer (plus ssh encoding / decoding, possibly rendering). You should only retrieve 100 rows, that would solve most of it, even if it means to execute the whole query every time.

If you go the route with a materialized view like I proposed you could add a serial number without gaps to the table plus index on it - by adding a column row_number() OVER (<your sort citeria here>) AS mv_id.

Then you can query:

SELECT *
FROM   materialized_view
WHERE  mv_id >= 2700
AND    mv_id <  2800;

This will perform very fast. LIMIT / OFFSET cannot compete, that needs to compute the whole table before it can sort and pick 100 rows.

pgAdmin timing

When you execute a query from the query tool, the message pane shows something like:

Total query runtime: 62 ms.

And the status line shows the same time. I quote pgAdmin help about that:

The status line will show how long the last query took to complete. If a dataset was returned, not only the elapsed time for server execution is displayed, but also the time to retrieve the data from the server to the Data Output page.

If you want to see the time on the server you need to use SQL EXPLAIN ANALYZE or the built in Shift + F7keyboard shortcut or Query -> Explain analyze. Then, at the bottom of the explain output you get something like this:

Total runtime: 0.269 ms

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One approach could be to use a writable view to transform results in-query, eg

ALTER TABLE utilities.labpreferences
    RENAME TO utilities.old_labpreferences

CREATE TABLE utilities.labpreferences
( utilities.old_labpreferences EXCLUDING DEFAULTS )
WITHOUT OIDS;

CREATE OR REPLACE RULE "labpreferences_select" AS
  ON SELECT TO utilities.labpreferences
  DO INSTEAD
  SELECT -- fields with modifications --
    FROM utilities.old_labpreferences
    WHERE s.id like 'My.%'
  UNION
  SELECT -- fields without modification --
    FROM utilities.old_labpreferences
    WHERE s.id not like 'My.%'
    ;

Backing the changes out is as simple as removing the rule and new (empty) table, and renaming the old table back.

You will also need rules for update/insert/delete to this table - the postgres documentation for DO INSTEAD is pretty good on this front - see http://www.postgresql.org/docs/devel/static/rules-views.html

Best Answer

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