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(at)oss(dot)nttdata(dot)com>
Reviewed-by: Jim Jones <jim(dot)jones(at)uni-muenster(dot)de>
Discussion: https://postgr.es/m/3520c224c5ffac0113aef84a9179f37e@oss.nttdata.com

Branch
------
master

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

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