How can I determine if a table exists in the current search_path with PLPGSQL?

Question

I'm writing a setup script for an application that's an addon for another application, so I want to check if the tables for the other application exist. If not, I want to give the user a useful error. However, I don't know what schema will be holding the tables.

DO LANGUAGE plpgsql $$
BEGIN
    PERFORM 1
    FROM
        pg_catalog.pg_class c
        JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
    WHERE
        n.nspname = current_setting('search_path')
        AND c.relname = 'foo'
        AND c.relkind = 'r'; -- not sure if I actually need this or not...

    IF NOT FOUND THEN
        RAISE 'This application depends on tables created by another application';
    END IF;
END;
$$;

However, current_setting('search_path') returns a TEXT containing "$user",public by default, which isn't terribly useful.

The only other thing I can think of is to try selecting from the table and catch the exception. It would do the job, but I don't think it is very elegant and I've read that it is expensive to use (though maybe that would be ok in this scenario since I'm only running it once?).

Answer 1

Quick and dirty

In Postgres 9.4+ use

SELECT to_regclass('foo');

Returns NULL if the identifier is not found in the search path.
In Postgres 9.3 or older use a cast to regclass:

SELECT 'foo'::regclass;

This raises an exception, if the object is not found!

If 'foo' is found, the oid is returned in its text representation. That's just the table name, schema-qualified according to the current search path and double-quoted where necessary.

If the object is not found you can be sure it does not exist anywhere in the search path - or not at all for a schema-qualified name (schema.foo).

If it's found there are two shortcomings:

1. The search includes implicit schemas of the search_path, namely pg_catalog and pg_temp. But you may want to exclude temp and system tables for your purpose. (?)

2. A cast to regclass works for all objects in the system catalog pg_class: indexes, views, sequences etc. Not just tables. You seem to be looking for a regular table exclusively. However, you'll probably have problems with other objects of the same name, too. Details:

   • How to check if a table exists in a given schema

Slow and sure

We are back to your query, but don't use current_setting('search_path'), which returns the bare setting. Use the dedicated system information function current_schemas(). Per documentation:

current_schemas(boolean) name[]
names of schemas in search path, optionally including implicit schemas

"$user" in the search path is resolved smartly. If no schema with the name of SESSION_USER exists, the schema is not returned to begin with. Also, depending on what you want exactly, you can additionally output implicit schemas (pg_catalog and possibly pg_temp) - but I assume you don't want those for the case at hand, so use:

DO 
$do$
BEGIN
   IF EXISTS (
      SELECT  -- list can be empty
      FROM   pg_catalog.pg_class c
      JOIN   pg_catalog.pg_namespace n ON n.oid = c.relnamespace
      WHERE  n.nspname = ANY(current_schemas(FALSE))
      AND    n.nspname NOT LIKE 'pg_%'  -- exclude system schemas!
      AND    c.relname = 'foo'
      AND    c.relkind = 'r')           -- you probably need this
   THEN
      RAISE 'This application depends on tables created by another application';
   END IF;
END
$do$;

SQL Fiddle, demonstrating all except for the last DO statement.
_{SQL Fiddle (JDBC) has problems with DO statements containing termination characters.}

Answer 2

You can convert the config value to an array and replace the $user with the current user name. The array can then be used in the where condition:

where n.nspname = any(string_to_array(replace(current_setting('search_path'), '$user', current_user), ','))

Answer 3

./sshi.sh vb20deployment controller <<'HERE'
export PGPASSWORD="postgres"
cd logu/postgresql/bin
row=1
tableArray=(table1 table2 table3 table4 table5 table6)

for (( x=0 ; x<=5 ; x++)) ; do        

./psql.bin --port=5432 --username=postgres --host=hostname.rds.amazonaws.com --dbname=mydb -c "SELECT * FROM information_schema.tables WHERE '${tableArray[$x]}' = table_name" | while read -a Record ; do
  row=$((row + 1))
  if [[ $row -gt 3 ]]; then

     echo ${Record[4]}

   fi
done

done


HERE