Re: pgbench: \gset and \aset should store SQL NULL as the null value

Hi,

I tested v2 on master (86f7c82cf10). It builds cleanly and the pgbench
TAP tests pass. Four tests in that file fail on my machine, but they
fail without the patch too.

I confirmed the bug. Neither --continue-on-error nor --max-tries helps.
The old behavior was broken for text substitution as well:
"SELECT 1 WHERE :nv IS NULL;" becomes "WHERE IS NULL" and fails. So I
do not think anyone relies on it.

One doc gap: 0001 changes what \gset does with a NULL column, but the \gset
docs say nothing about NULL. One sentence is enough, so no v3 is
needed. Master only, no back-patch.

Please commit 0002 together with 0001. Patch 0001 says a captured NULL
can be tested with IS NULL, but that test cannot tell a real NULL from
a column holding the text 'NULL'. Only 0002 explains this.

As CommitFest manager I am moving
https://commitfest.postgresql.org/patch/7163/ to Ready for Committer,

Thanks,
Shihao