PL/pgSQL perform vs execute

PERFORM is plpgsql command used for calls of void functions. PLpgSQL is careful about useless SELECT statements – the SELECT without INTO clause is not allowed. But sometimes you need to call a function and you don’t need to store result (or functions has no result). The function in SQL is called with SELECT statement. But it is not possible in PLpgSQL – so the command PERFORM was introduced.

CREATE OR REPLACE FUNCTION foo()
RETURNS void AS $$
BEGIN
  RAISE NOTICE 'Hello from void function';
END;
$$ LANGUAGE plpgsql;

-- direct call from SQL
SELECT foo();

-- in PLpgSQL
DO $$
BEGIN
  SELECT foo(); -- is not allowed
  PERFORM foo(); -- is ok
END;
$$;

The PERFORM statements execute a parameter and forgot result.

Your example perform 'create table foo as (select 1)';

is same like SELECT 'create table foo as (select 1)'. It returns a string “create table foo as (select 1)” and this string is discarded.

The EXECUTE statement evaluate a expression to get string. In next step this string is executed.

So EXECUTE 'create table ' || some_var || '(a int)'; has two steps

1. evaluate expression 'create table ' || some_var || '(a int)'
2. if some_var is mytab for example, then execute a command create table mytab(a int)

The PERFORM statement is used for function calls, when functions are not used in assignment statement. The EXECUTE is used for evaluation of dynamic SQL – when a form of SQL command is known in runtime.