Re: PG8.3->10 migration data differences

On 2018-09-10 11:57:42 +0300, Achilleas Mantzios wrote:
> On 10/09/2018 11:22, Csaba Ragasits wrote:
>     Hello,
>
>     We would like to migrate our old databases to new postgres server,
>     with the simple backup-restore process. The process run fine
>     without errors but when we're  checking the source and the
>     migrated datas as field level, we found the following differences:
[...]
>
>     Field type: REAL
>     pg93: 2.2
>     pg10: 2.20000005
>
>
> Those have to do with rounding. Precision for real is 6 decimal
> digits. Your difference is on the 8-th digit.

Nitpick: Precision is 24 binary digits which is *about* 6 decimal
digits. The number which is actually stored (on both pg93 and pg10) is
actually 2.2000000476837158203125 (1.00011001100110011001101 * 2^1 in
binary). Apparently Pg9.3 rounds this to "2.2" on output while Pg10
prints the more precise (but still not exact) "2.20000005".

(I would argue that the Pg9.3 output is better, since it represents the
same value in fewer digits, but always printing the minimum number of
digits necessary is surprisingly difficult.)

        hp

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