Which data type is the result of division？

List the round function in postgres:

\df  round
                         List of functions
   Schema   | Name  | Result data type | Argument data types | Type 
------------+-------+------------------+---------------------+------
 pg_catalog | round | double precision | double precision    | func
 pg_catalog | round | numeric          | numeric             | func
 pg_catalog | round | numeric          | numeric, integer    | func

The argument for round must be double precision,numeric,integer.

select  6::float/3.3 as number;
       number       
--------------------
 1.8181818181818183
(1 row)

If its data type is double precision :

select  round(6::float/3.3,4) as number;
ERROR:  function round(double precision, integer) does not exist
LINE 1: select  round(6::float/3.3,4) as number;
                ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
Time: 0.517 ms

Which data type is the result of division—-1.8181818181818183？

>Solution :

You can always check things like that with pg_typeof(): demo

select pg_typeof(6::float/3.3) as number_type;

The reason your function call didn’t work is that \df showed you three different round() functions:

round ( numeric ) → numeric

round ( double precision ) → double precision

Rounds to nearest integer. For numeric, ties are broken by rounding away from zero. For double precision, the tie-breaking behavior is platform dependent, but “round to nearest even” is the most common rule.

and only the one accepting numeric has a variant with a 2nd argument, integer, to specify how
many decimal places you want to round to.

round ( v numeric, s integer ) → numeric

Rounds v to s decimal places. Ties are broken by rounding away from zero.

So if you cast to ::numeric instead or don’t cast all and let PostgreSQL assume numeric by default, you can get that one to get picked:

select round(6/3.3,4) as number;