ADQL-2.0 Erratum 2 - MOD function description

IVOA DAL mailing list thread on July 2016 highlighted a possible issue with the description of the MOD function.

Erratum Content

Table 1 on page 8 of the ADQL-2.0 recommendation reports

Name	Argument data type	Return data type	Description
mod(x, y)	double	double	Returns the remainder of y/x.

This seems to conflict with SQL (92, 2003) definitions and, as well, the behaviour of the existing TAP services, suggesting that x and y should be swapped in the function description.

Rationale

Considering what current TAP services using ADQL and the mathematical description of the modulo operator where

M % N = R

with

• R having the same sign as M
• |R| is less than |N|
• M = K * N + R for a given integer K

The definition of mod(x, y) in ADQL-2.0 should be amended to "Returns the remainder of x/y". It should also be made clear, maybe in a future revision, to follow the above rules for the remainder and its sign.

Impact Assesment

The change proposed by this erratum should impact no existing IVOA resource. Checks against the major TAP services shows they already act following the behaviour here above described. Checks against a choice of RDBMS-es (PostgreSQL, MySQL, MariaDB, Apache Derby, HyperSQL and Oracle) shows the same bahaviour too. See the mail thread referenced in the introduction of this page for details.