Setting the number of digits after the decimal point with a psql's pset variable

Hello,

Each time I execute a query to get some ratio or percentage, I end up
with a lot of usually useless digits after the decimal point. For example:

postgres=> select datname,
100.*blks_hit/(blks_read+blks_hit) as hit_ratio
from pg_stat_database
order by 1
limit 5;

datname | hit_ratio
---------+---------------------
b1 | 70.0758884178573298
b2 | 97.3346067025525265
b3 | 97.8544316239962496
b4 | 31.8874531850956484
b5 | 43.0444853056359762
(5 rows)

I would rather get only two digits (or one, or even less) after the
decimal point. I know I can do that easily by wrapping the second column
with a call to round(). Easily because this query is simple. But
sometimes, the query is much bigger, the number of columns which need a
call to round() is high enough to make me stop doing it or get pissed
because it gets quickly error prone. I need something quicker and
easier. Something like a pset variable which would allow me to tell psql
how many digits I want after the decimal point. The patch attached does
that. It adds a pset variable called n_decimals, which defaults to all,
meaning no changes in psql's behaviour. A user can set it to any
positive number to get that number of digits after the decimal point. An
example:

postgres=> \pset n_decimals 2
Number of decimals is "2".

postgres=> \g

datname | hit_ratio
---------+-----------
b1 | 70.10
b2 | 97.33
b3 | 97.85
b4 | 31.89
b5 | 43.05
(5 rows)

postgres=> \pset n_decimals 0
Number of decimals is "0".

postgres=> \g

datname | hit_ratio
---------+-----------
b1 | 70
b2 | 97
b3 | 97
b4 | 31
b5 | 43
(5 rows)

postgres=> \pset n_decimals
Number of decimals is "0".

postgres=> \pset n_decimals all
Number of decimals is "all".

postgres=> \g

datname | hit_ratio
---------+---------------------
b1 | 70.1052134669652421
b2 | 97.3396221743544034
b3 | 97.8583698372493909
b4 | 31.8998905599223468
b5 | 43.0615702842088872
(5 rows)

Any comments welcome :)

Thank you.

Regards.

--
Guillaume Lelarge
Consultant
https://dalibo.com