how to implement contains operator in Postgres - postgresql

Question

How to implement contains operator for strings which returns true if left string contains in right string.
Operator name can be any. I tried ## and code below but
select 'A' ## 'SAS'
returns false.
How to fix ?
CREATE OR REPLACE FUNCTION public.contains(searchFor text, searchIn text)
RETURNS bool
AS $BODY$ BEGIN
RETURN position( searchFor in searchIn)<>0;
END; $BODY$ language plpgsql immutable RETURNS NULL ON NULL INPUT;
CREATE OPERATOR public.## (
leftarg = text,
rightarg = text,
procedure = public.contains
);
Using Postgres 9.1 and above in windows and linux.
select contains('A' , 'SAS' )
returns true as expected.
Update
I tried in 9.1 code from answer:
CREATE OR REPLACE FUNCTION public.contains(searchFor text, searchIn text)
RETURNS bool
LANGUAGE sql IMMUTABLE
AS $BODY$
SELECT position( searchFor in searchIn )<>0;
$BODY$;
CREATE OPERATOR public.<# (
leftarg = text,
rightarg = text,
procedure = public.contains
);
but got error
ERROR: column "searchin" does not exist
LINE 5: SELECT position( searchFor in searchIn )<>0;
How to make it work in 9.1 ? In 9.3 it works.
Using
"PostgreSQL 9.1.2 on x86_64-unknown-linux-gnu, compiled by gcc-4.4.real (Debian 4.4.5-8) 4.4.5, 64-bit"

Answer 1

PostgreSQL already defines an ## operator on (text,text) in the pg_catalog schema, which is defined as:
regress=> \do ##
List of operators
Schema | Name | Left arg type | Right arg type | Result type | Description
------------+------+---------------+----------------+-------------+-------------------
pg_catalog | ## | text | text | boolean | text search match
That is taking precedence over the ## you defined in the public schema.
I suggest using the operator <# for contains, because that's consistent with the array contains and contained-by operators. There's no usage of it in pg_catalog for text ... but there's no guarantee one won't be added in a future version or by an extension.
If you want to guarantee that your operators take precedence over those in pg_catalog, you need to put them in a different schema and put it first on the search path, explicitly before pg_catalog. So something like:
CREATE SCHEMA my_operators;
CREATE OR REPLACE FUNCTION my_operators.contains(searchFor text, searchIn text)
RETURNS bool
LANGUAGE sql IMMUTABLE
AS $BODY$
SELECT position( searchFor in searchIn)<>0;
$BODY$;
CREATE OPERATOR my_operators.<# (
leftarg = text,
rightarg = text,
procedure = public.contains
);
then
SET search_path = 'my_operators, pg_catalog, public';
which you can do with ALTER USER and/or ALTER DATABASE to make it a default.