Allow COPY FROM to filter data using WHERE conditions

Allow COPY FROM to filter data using WHERE conditions

Extends the COPY FROM command with a WHERE condition, which allows doing
various types of filtering while importing the data (random sampling,
condition on a data column, etc.).  Until now such filtering required
either preprocessing of the input data, or importing all data and then
filtering in the database. COPY FROM ... WHERE is an easy-to-use and
low-overhead alternative for most simple cases.

Author: Surafel Temesgen
Reviewed-by: Tomas Vondra, Masahiko Sawada, Lim Myungkyu
Discussion: https://www.postgresql.org/message-id/flat/CALAY4q_DdpWDuB5-Zyi-oTtO2uSk8pmy+dupiRe3AvAc++1imA@mail.gmail.com