pgsql: Add HINT for COPY TO when WHERE clause is used.

Add HINT for COPY TO when WHERE clause is used.

COPY TO does not support a WHERE clause, and currently fails with the error:

    ERROR:  WHERE clause not allowed with COPY TO

Since the intended behavior can be achieved by using
COPY (SELECT ... WHERE ...) TO, this commit adds a HINT
to the error message:

    HINT:  Try the COPY (SELECT ... WHERE ...) TO variant.

This makes the error more informative and helps users
quickly find the alternative usage.

Author: Atsushi Torikoshi <torikoshia@oss.nttdata.com>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Discussion: https://postgr.es/m/3520c224c5ffac0113aef84a9179f37e@oss.nttdata.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/229911c4bf9a4acfb2b81cff148fd8391e0a577b

Modified Files
--------------
src/backend/parser/gram.y           | 1 +
src/test/regress/expected/copy2.out | 1 +
2 files changed, 2 insertions(+)