This is documented in section 8.1.2 in the manual. ( https://www.postgresql.org/docs/9.6/static/datatype-numeric.html)
NUMERIC rounds away from zero. IEEE 754 based data types (FLOAT, DOUBLE PRECISION) round to the closest even number. On Thu, Jun 1, 2017 at 6:26 PM, Louis Battuello <louis.battue...@etasseo.com > wrote: > Is the round() function implemented differently for double precision than > for numeric? Forgive me if this exists somewhere in the documentation, but > I can't seem to find it. > > I've noticed with 9.6 on OSX, the .5 rounding is handled differently > between the types. (I haven't tested other versions, yet.) For double > precision values, even whole numbers are rounded down, yet for odds they > are rounded up. For numeric values, all .5 numbers are rounded up. > > psql (9.6.3) > Type "help" for help. > > postgres=# \x > Expanded display is on. > postgres=# select round(cast(1230.5 as double precision)) as > round_double_even_0 > postgres-# ,round(cast(1231.5 as double precision)) as > round_double_odd_1 > postgres-# ,round(cast(1232.5 as double precision)) as > round_double_even_2 > postgres-# ,round(cast(1233.5 as double precision)) as > round_double_odd_3 > postgres-# ,round(cast(1234.5 as double precision)) as > round_double_even_4 > postgres-# ,round(cast(1235.5 as double precision)) as > round_double_odd_5 > postgres-# ; > > -[ RECORD 1 ]-------+----- > round_double_even_0 | 1230 > round_double_odd_1 | 1232 > round_double_even_2 | 1232 > round_double_odd_3 | 1234 > round_double_even_4 | 1234 > round_double_odd_5 | 1236 > > postgres=# select round(cast(1230.5 as numeric)) as round_numeric_even_0 > postgres-# ,round(cast(1231.5 as numeric)) as round_numeric_odd_1 > postgres-# ,round(cast(1232.5 as numeric)) as round_numeric_even_2 > postgres-# ,round(cast(1233.5 as numeric)) as round_numeric_odd_3 > postgres-# ,round(cast(1234.5 as numeric)) as round_numeric_even_4 > postgres-# ,round(cast(1235.5 as numeric)) as round_numeric_odd_5 > postgres-# ; > > -[ RECORD 1 ]--------+----- > round_numeric_even_0 | 1231 > round_numeric_odd_1 | 1232 > round_numeric_even_2 | 1233 > round_numeric_odd_3 | 1234 > round_numeric_even_4 | 1235 > round_numeric_odd_5 | 1236 > > postgres=# select round(1230.5) as round_even_0 > ,round(1231.5) as round_odd_1 > ,round(1232.5) as round_even_2 > ,round(1233.5) as round_odd_3 > ,round(1234.5) as round_even_4 > ,round(1235.5) as round_odd_5 > ; > > -[ RECORD 1 ]+----- > round_even_0 | 1231 > round_odd_1 | 1232 > round_even_2 | 1233 > round_odd_3 | 1234 > round_even_4 | 1235 > round_odd_5 | 1236 > > postgres=# \q > > Why does the algorithm vary by data type? > > Or is something entirely different happening? > >
