PL/pgSQL checking if a row exists

Question

I'm writing a function in PL/pgSQL, and I'm looking for the simplest way to check if a row exists.
Right now I'm SELECTing an integer into a boolean, which doesn't really work. I'm not experienced with PL/pgSQL enough yet to know the best way of doing this.

Here's part of my function:

DECLARE person_exists boolean; BEGIN  person_exists := FALSE;  SELECT "person_id" INTO person_exists   FROM "people" p WHERE p.person_id = my_person_id LIMIT 1;  IF person_exists THEN   -- Do something END IF;  END; $$ LANGUAGE plpgsql; 

Update - I'm doing something like this for now:

DECLARE person_exists integer; BEGIN  person_exists := 0;  SELECT count("person_id") INTO person_exists   FROM "people" p WHERE p.person_id = my_person_id LIMIT 1;  IF person_exists < 1 THEN   -- Do something END IF; 

Answer 1

Simpler, shorter, faster: EXISTS.

IF EXISTS (SELECT 1 FROM people p WHERE p.person_id = my_person_id) THEN   -- do something END IF; 

The query planner can stop at the first row found - as opposed to count(), which will scan all (matching) rows regardless. Makes a difference with big tables. The difference is small for a condition on a unique column: only one row qualifies and there is an index to look it up quickly.

^{Improved with input from @a_horse_with_no_name in the comments.}

You can just use an empty SELECT list:

IF EXISTS (SELECT FROM people p WHERE p.person_id = my_person_id) THEN ... 

The SELECT list has no influence on the result of EXISTS. Only the existence of at least one qualifying row matters.

Answer 2

Use count(*)

declare     cnt integer; begin   SELECT count(*) INTO cnt   FROM people   WHERE person_id = my_person_id;  IF cnt > 0 THEN   -- Do something END IF; 

Edit (for the downvoter who didn't read the statement and others who might be doing something similar)

The solution is only effective because there is a where clause on a column (and the name of the column suggests that its the primary key - so the where clause is highly effective)

Because of that where clause there is no need to use a LIMIT or something else to test the presence of a row that is identified by its primary key. It is an effective way to test this.