How to truncate double precision value in PostgreSQL by keeping exactly first two decimals? - json

Question

I'm trying to truncate double precision value when I'm build json using json_build_object() function in PostgreSQL 11.8 but with no luck. To be more precise I'm trying to truncate 19.9899999999999984 number to ONLY two decimals but making sure it DOES NOT round it to 20.00 (which is what it does), but to keep it at 19.98.
BTW, what I've tried so far was to use:
1) TRUNC(found_book.price::numeric, 2) and I get value 20.00
2) ROUND(found_book.price::numeric, 2) and I get value 19.99 -> so far this is closesest value but not what I need
3) ROUND(found_book.price::double precision, 2) and I get
[42883] ERROR: function round(double precision, integer) does not exist
Also here is whole code I'm using:
create or replace function public.get_book_by_book_id8(b_id bigint) returns json as
$BODY$
declare
found_book book;
book_authors json;
book_categories json;
book_price double precision;
begin
-- Load book data:
select * into found_book
from book b2
where b2.book_id = b_id;
-- Get assigned authors
select case when count(x) = 0 then '[]' else json_agg(x) end into book_authors
from (select aut.*
from book b
inner join author_book as ab on b.book_id = ab.book_id
inner join author as aut on ab.author_id = aut.author_id
where b.book_id = b_id) x;
-- Get assigned categories
select case when count(y) = 0 then '[]' else json_agg(y) end into book_categories
from (select cat.*
from book b
inner join category_book as cb on b.book_id = cb.book_id
inner join category as cat on cb.category_id = cat.category_id
where b.book_id = b_id) y;
book_price = trunc(found_book.price, 2);
-- Build the JSON response:
return (select json_build_object(
'book_id', found_book.book_id,
'title', found_book.title,
'price', book_price,
'amount', found_book.amount,
'is_deleted', found_book.is_deleted,
'authors', book_authors,
'categories', book_categories
));
end
$BODY$
language 'plpgsql';
select get_book_by_book_id8(186);
How do I achieve to keep EXACTLY ONLY two FIRST decimal digits 19.98 (any suggestion/help is greatly appreciated)?
P.S. PostgreSQL version is 11.8

Answer 1

In PostgreSQL 11.8 or 12.3 I cannot reproduce:
# select trunc('19.9899999999999984'::numeric, 2);
trunc
-------
19.98
(1 row)
# select trunc(19.9899999999999984::numeric, 2);
trunc
-------
19.98
(1 row)
# select trunc(19.9899999999999984, 2);
trunc
-------
19.98
(1 row)
Actually I can reproduce with the right type and a special setting:
# set extra_float_digits=0;
SET
# select trunc(19.9899999999999984::double precision::text::numeric, 2);
trunc
-------
19.99
(1 row)
And a possible solution:
# show extra_float_digits;
extra_float_digits
--------------------
3
(1 row)
select trunc(19.9899999999999984::double precision::text::numeric, 2);
trunc
-------
19.98
(1 row)
But note that:
Note: The extra_float_digits setting controls the number of extra
significant digits included when a floating point value is converted
to text for output. With the default value of 0, the output is the
same on every platform supported by PostgreSQL. Increasing it will
produce output that more accurately represents the stored value, but
may be unportable.

Answer 2

As #pifor suggested I've managed to get it done by directly passing trunc(found_book.price::double precision::text::numeric, 2) as value in json_build_object like this:
json_build_object(
'book_id', found_book.book_id,
'title', found_book.title,
'price', trunc(found_book.price::double precision::text::numeric, 2),
'amount', found_book.amount,
'is_deleted', found_book.is_deleted,
'authors', book_authors,
'categories', book_categories
)
Using book_price = trunc(found_book.price::double precision::text::numeric, 2); and passing it as value for 'price' key didn't work.
Thank you for your help. :)