sql: Document COPY statement

[rafiss]

CockroachDB has supported the COPY statement for the past few releases, but it is not documented.

Code changes:
cockroachdb/cockroach#8756
cockroachdb/cockroach#51890

Other docs issues:
#546
#3058
#1676
#1831
#8156
#8155

[rafiss]

cc @ericharmeling

[ericharmeling]

@rafiss I'm a little confused here. cockroachdb/cockroach#16392 suggests that we do not support COPY FROM STDIN, but the grammar allows the syntax: https://github.com/cockroachdb/cockroach/blob/master/pkg/sql/parser/sql.y#L2859

[rafiss]

The distinction there is that the cockroach CLI tool does not support COPY statements, but if you have an app or some other tool that sends a COPY statement to a CockroachDB server, we do support that. (Though we don't support all of the COPY syntax that Postgres has. See cockroachdb/cockroach#41608)

[ericharmeling]

The distinction there is that the cockroach CLI tool does not support COPY statements, but if you have an app or some other tool that sends a COPY statement to a CockroachDB server, we do support that. (Though we don't support all of the COPY syntax that Postgres has. See cockroachdb/cockroach#41608)

I see. Thanks for the clarification!

This type of support seems fairly particular. Are there any other statements that you know of that are only supported if issued by some third-party clients?

[rafiss]

Hmm no, off the top of my head I'm not familiar of other syntax that we support in that way, but COPY is a rather odd/special statement.

[ericharmeling]

@rafiss Another question... do we only support executing COPY FROM (and not COPY TO)?

[rafiss]

Correct, just COPY FROM