Row count in TAP_SCHEMA

[Mark Taylor]

Hi DAL,

since VODataService v1.2 (see sec 3.3), the Table element has had
an optional attribute "nrows" which allows services to declare how
many rows a table has.  That is useful information, and TOPCAT
displays it, if known, as part of the table metadata in its TAP window.

However, there is currently no corresponding standard way to report
this information from TAP_SCHEMA.  Topcat sometimes gets TAP service
metadata from the /tables endpoint (VODataService) and sometimes from 
TAP_SCHEMA (depending on things like apparent service size); in the former
case it's able to report table sizes, but in the latter case it's not.

So it would be nice to have a standard way in which TAP services
could report table size in TAP_SCHEMA if they wanted to.
This would just need to be a new optional column with an agreed
name in TAP_SCHEMA.tables.  

In fact some services already do this, but different column names
are in use.  ARI-Gaia uses "row_count" and ESA uses "size"
(and also has "size_bytes" for size in bytes).  "size" is a
somewhat problematic choice since it's an ADQL reserved word,
it's also not very explicit about what it means.
"row_count" is OK by me, though "nrows" would also be reasonable
for consistency with VODataService.

Could we agree here on a suitable column name for this?  
Next time there's a TAP update it could go in there, 
but there's nothing to stop people agreeing on and implementing 
best practice in the mean time; since the column would be optional,
and you're allowed to add non-standard columns in TAP_SCHEMA,
it doesn't break anything.

Mark

--
Mark Taylor  Astronomical Programmer  Physics, Bristol University, UK
m.b.taylor at bristol.ac.uk          http://www.star.bristol.ac.uk/~mbt/