Sequence name taken from variable - sql

Question

How do I create a new sequence taking its name is from a variable?
Let's take a look at the following example:
CREATE OR REPLACE FUNCTION get_value(_name_part character varying)
RETURNS INTEGER AS
$BODY$
DECLARE
result bigint;
sequencename character varying(50);
BEGIN
sequencename = CONCAT('constant_part_of_name_', _name_part);
IF((SELECT CAST(COUNT(*) AS INTEGER) FROM pg_class
WHERE relname LIKE sequencename) = 0)
THEN
CREATE SEQUENCE sequencename --here is the guy this is all about
MINVALUE 6000000
INCREMENT BY 1;
END IF;
SELECT nextval(sequencename) INTO result;
RETURN result;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
Now, let's say I want a sequence with _name_part = 'Whatever', so I type:
SELECT get_value('Whatever');
If sequence constant_part_of_name_Whatever does not exist, my function should create it and take a value; if it exists it should only take a value. However, I created sequence constant_part_of_name_sequencename.
How do I put the value of the variable in sequence definition to make it work?

Answer 1

The currently accepted answer has a number of problems. Most importantly it fails to take the schema into account.
Use instead:
CREATE OR REPLACE FUNCTION get_value(_name_part text)
RETURNS bigint AS
$func$
DECLARE
_seq text := 'constant_part_of_name_' || _name_part;
BEGIN
CASE (SELECT c.relkind = 'S'::"char"
FROM pg_namespace n
JOIN pg_class c ON c.relnamespace = n.oid
WHERE n.nspname = current_schema() -- or provide your schema!
AND c.relname = _seq)
WHEN TRUE THEN -- sequence exists
-- do nothing
WHEN FALSE THEN -- not a sequence
RAISE EXCEPTION '% is not a sequence!', _seq;
ELSE -- sequence does not exist, name is free
EXECUTE format('CREATE SEQUENCE %I MINVALUE 6000000 INCREMENT BY 1', _seq);
END CASE;
RETURN nextval(_seq);
END
$func$ LANGUAGE plpgsql;
SQL Fiddle.
Major points
Your test was needlessly expensive and incorrect. You need to take the schema into account. A sequence of the same name can exist in another schema, which would make your function fail.
I use the current schema as default, since you did not specify otherwise. Details:
How does the search_path influence identifier resolution and the "current schema"
You also need to be aware that the name of a sequence conflicts with other names of other objects in the same schema. Details:
How to create sequence if not exists
varchar(50) as data type is pointless and may cause problems if you enter a longer string. Just use text or varchar.
The assignment operator in plpgsql is :=, not =.
You can assign a variable at declaration time. Shorter, cheaper, cleaner.
You need dynamic SQL, I am using format() with %I to escape the identifier properly. Details:
INSERT with dynamic table name in trigger function
concat() is only useful if NULL values can be involved. I assume you don't want to pass NULL.
VOLATILE is default and therefore just noise.
If you want to return NULL on NULL input, add STRICT.

Answer 2

Try this. Hope this work for you.
CREATE OR REPLACE FUNCTION get_value(_name_part character varying) RETURNS INTEGER AS
$BODY$
DECLARE
result bigint;
sequencename character varying(50);
v_sql character varying;
BEGIN
sequencename = CONCAT('constant_part_of_name_', _name_part);
IF((SELECT CAST(COUNT(*) AS INTEGER) FROM pg_class WHERE relname LIKE sequencename) = 0)
THEN
v_sql := 'CREATE SEQUENCE '||sequencename||'
MINVALUE 6000000
INCREMENT BY 1;';
EXECUTE v_sql;
END IF;
SELECT nextval(sequencename) INTO result ;
RETURN result;
END;
$BODY$
LANGUAGE plpgsql VOLATILE

Answer 3

CREATE OR REPLACE FUNCTION get_value(_name_part character varying) RETURNS INTEGER AS
$BODY$
DECLARE
result bigint;
sequencename character varying(50);
BEGIN
sequencename = CONCAT('constant_part_of_name_', _name_part);
IF (select exists(SELECT relname FROM pg_class c WHERE c.relkind = 'S' and relname = ''''||sequencename||'''') = false )
THEN
execute 'CREATE SEQUENCE '||sequencename||'MINVALUE 6000000 INCREMENT BY 1';
else
END IF;
execute 'SELECT nextval('''||sequencename||''')' INTO result;
RETURN result;
END;
$BODY$
LANGUAGE plpgsql VOLATILE