PostgreSQL CAST() giving unexpected results

Question

I can't seem to figure out how this is rounding overflow digits. It is most definitely not any of:

• Round up if last digit is 5 or higher.
• Round to even if last digit is 5.
• Randomly round up or down if last digit is 5.

In fact, whatever logic it is using is definitely 1) deterministic, and 2) not [completely] based on the last digit being 5.

Some example inputs and output of:

CAST ('input' AS DOUBLE PRECISION);

Here, 'input' is not a variable, just a placeholder in this post for a literal numeric value with quotes around it, e.g. below in input column.

input output
34.55555555555595 34.5555555555559 *Obviously not rounding up
34.555555555555951 34.5555555555559 *Why not round up here?
34.5555555555559514 34.5555555555559 *Another unexpected round down
34.5555555555559515 34.555555555556 *Unexpected/seemingly inconsistent
34.55555555555595145 34.5555555555559
34.55555555555595146 34.5555555555559
34.55555555555595148 34.5555555555559
34.55555555555595149 34.555555555556 *Huh!?

Version: PostgreSQL 9.3.13 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2, 64-bit

• user1822: CAST ('x' AS DOUBLE PRECISION); will not work at all. It will result in the error: invalid input syntax for type double precision: "x" and will not return any values at all.
• ypercubeᵀᴹ: From Postgres docs: Numeric types: "double precision, 8 bytes, variable-precision, inexact, 15 decimal digits precision" You are passing more than 15, even 18 digits to a type that can hold 15-16 digits. What did you expect?
• ypercubeᵀᴹ: From the same link: "Inexact means that some values cannot be converted exactly to the internal format and are stored as approximations, so that storing and retrieving a value might show slight discrepancies."
• ypercubeᵀᴹ: Necessary read: What Every Programmer Should Know About Floating-Point Arithmetic
• Jim: a_horse_with_no_name: I was using 'x' as a symbol within the post itself, not as a variable. I updated the question to be more clear.

Answer 1

Based on your last comment,

JavaScript parseFloat("34.55555555555595149").toPrecision(15)

the cast should be to the numeric datatype

CAST ('input' AS numeric(17,15))

https://www.postgresql.org/docs/8.1/static/datatype.html#DATATYPE-NUMERIC-DECIMAL