Alternative to \copy in psql modelled after \g

Hi,

Currently \copy cannot span multiple lines (like any meta-command)
and cannot use psql variables whereas \g can do both.

The POC patch attached implements two meta-commands \copyfrom
and \copyto that are to COPY what \g is to any other query:

- they take the COPY query already var-expanded from the query buffer,
which must mention FROM STDIN or TO STDOUT.

- they accept an argument declaring the local data source or destination,
either a filename or a program (|command args) or empty for stdin/stdout.

By contrast \copy has a specific parser to extract the data source
or dest from its line of arguments, plus whether it's a COPY FROM or TO,
and build a COPY query from that.

Examples of use

1. $ psql -v filename="/path/data-$(date -I).csv"
COPY (SELECT *
FROM table
WHERE ...)
TO STDOUT (FORMAT csv) \copyto :filename

2. -- copy only the first 100 lines
COPY table FROM stdin \copyfrom |head -n 100 /data/datafile.txt

3. $ cat script.sql
COPY table1 FROM stdin; -- copy inline data
data line
data line
\.

-- copy data from psql's stdin
COPY table2 FROM stdin \copyfrom

# copy both in-script and out-of-script data
$ psql -f script.sql < table2.data

Comments? Does that look useful as an alternative to \copy ?

Best regards,
--
Daniel Vérité
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