"perform create index" in plpgsql doesn't run - postgresql

Question

I'm having problems executing a "perform create index" inside of a plgpsql function (postgres 9.4). For example:
create or replace function foo() returns void language plpgsql as $$
begin
perform 'create unique index patients_row_id_key on patients(row_id)';
end; $$;
It seems to run fine:
select foo();
However, the index is not created. Any diagnosis and workaround? I tried:
alter function foo() VOLATILE;
and still no luck.

Answer 1

What #Abelisto wrote about PERFORM.
And what #Chris added about SQL injection.
Plus, I suggest to use format() for anything except the most trivial query strings to make your life with dynamic SQL easier. And the manual does, too:
A cleaner approach is to use format()'s %I specification for table or column names.
CREATE OR REPLACE FUNCTION foo(_tbl text)
RETURNS void AS
$func$
BEGIN
EXECUTE format('CREATE UNIQUE INDEX %I ON %I(row_id)', _tbl || _row_id_key', _tbl);
END
$func$ LANGUAGE plpgsql;
A regclass parameter is a convenient alternative for passing table names, but concatenating new identifiers can be tricky - as this recent related case goes to show:
PL/pgSQL regclass quoting of table named like keyword

Answer 2

As a supplement to the point of using execute, note two important points about this.
You are doing string interpolation with sql queries (dangerous!), and
You have to use quote_ident, not quote_literal
If you use Abelisto's function above, and call it with:
SELECT foo('test_idx on test; drop table foo; --');
SQL injection in stored procedure. Worse if it is security definer. A fixed version would be:
create or replace function foo(p_tablename text) returns void language plpgsql as $$
begin
execute 'create unique index ' || quote_ident(p_tablename || '_row_id_key') || ' on ' || quote_ident(p_tablename) || '(row_id)';
end; $$;

Answer 3

PERFORM statement in the PLPGSQL used to execute queries which does not return result or which result is not useful. Technically PERFORM ... inside the PLPGSQL block is equal to SELECT ... in the plain SQL. So in your example you are trying to execute something like
select 'create unique index patients_row_id_key on patients(row_id)';
and just ignore the result.
Read more: Executing a Command With No Result
You should not to wrap DDL statements inside PLPGSQL and can use it as is:
create or replace function foo() returns void language plpgsql as $$
begin
create unique index patients_row_id_key on patients(row_id);
end; $$;
Or if you want to construct it at runtime then use EXECUTE statement: Executing Dynamic Commands like this:
create or replace function foo(p_tablename text) returns void language plpgsql as $$
begin
execute 'create unique index ' || p_tablename || '_row_id_key on ' || p_tablename || '(row_id)';
end; $$;