How to get the number of days in a month?

Using the smart "trick" to extract the day part from the last date of the month, as demonstrated by Quassnoi. But it can be a bit simpler / faster:

SELECT extract(days FROM date_trunc('month', now()) + interval '1 month - 1 day');

Rationale

extract is standard SQL, so maybe preferable, but it resolves to the same function internally as date_part(). The manual:

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

But we only need to add a single interval. Postgres allows multiple time units at once. The manual:

interval values can be written using the following verbose syntax:

[@] quantity unit[quantity unit...] [direction]

where quantity is a number (possibly signed); unit is microsecond, millisecond, second, minute, hour, day, week, month, year, decade, century, millennium, or abbreviations or plurals of these units;

ISO 8601 or standard SQL format are also accepted. Either way, the manual again:

Internally interval values are stored as months, days, and seconds. This is done because the number of days in a month varies, and a day can have 23 or 25 hours if a daylight savings time adjustment is involved. The months and days fields are integers while the seconds field can store fractions.

(Output / display depends on the setting of IntervalStyle.)

The above example uses default Postgres format: interval '1 month - 1 day'. These are also valid (while less readable):

interval '1 mon - 1 d' -- unambiguous abbreviations of time units are allowed

IS0 8601 format:

interval '0-1 -1 0:0'

Standard SQL format:

interval 'P1M-1D';

All the same.

You can write a function:

CREATE OR REPLACE FUNCTION get_total_days_in_month(timestamp)
RETURNS decimal
IMMUTABLE
AS $$
  select cast(datediff(day, date_trunc('mon', $1), last_day($1) + 1) as decimal)
$$ LANGUAGE sql;