DOCS: Tip and Tricks for Conesearch via TAP

[agy-why]

When an indexed pos column is present in a table, the user should prefer the Postgres ConeSearch variant over the ADQL ConseSearch variant because the first one make use of the indexed column.

The translation of he ADQL query into a Postgres query can not assume that the pos column exists and as a results, uses the ra and dec values, preventing the use of the indexed column.

Finally in case of gal_ra and gal_dec, a simple convertion into equatorial coord allows the use of pos:

SELECT * FROM <gis_table> 
WHERE pos @ scircle(gal2eq(gal_ra, gal_dec), radius)

we could decide to provide such functions:

• gal2eq: galactic coord to equatorial coord
• eq2gal: equatorial coord to galactic coord
• ...