Re: psql: Add command to use extended query protocol

>
>
> Btw., this also allows doing things like
>
> SELECT $1, $2
> \bind '1' '2' \g
> \bind '3' '4' \g
>

That's one of the things I was hoping for. Very cool.

>
> This isn't a prepared statement being reused, but it relies on the fact
> that psql \g with an empty query buffer resends the previous query.
> Still kind of neat.

Yeah, if they wanted a prepared statement there's nothing stopping them.

Review:

Patch applies, tests pass.

Code is quite straightforward.

As for the docs, they're very clear and probably sufficient as-is, but I
wonder if we should we explicitly state that the bind-state and bind
parameters do not "stay around" after the query is executed? Suggestions in
bold:

This command causes the extended query protocol (see <xref
linkend="protocol-query-concepts"/>) to be used, unlike normal
<application>psql</application> operation, which uses the simple
query protocol. *Extended query protocol will be used* *even if
no parameters are specified, s*o this command can be useful to test the
extended
query protocol from psql. *This command affects only the next
query executed, all subsequent queries will use the regular query protocol
by default.*

Tests seem comprehensive. I went looking for the TAP test that this would
have replaced, but found none, and it seems the only test where we exercise
PQsendQueryParams is libpq_pipeline.c, so these tests are a welcome
addition.

Aside from the possible doc change, it looks ready to go.