Re: Proposal to add --single-row to psql

Hello

It is redundant to current FETCH_COUNT implementation, so I don't see sense
to use it together. Maybe we can drop FETCH_COUNT and replace it by
--single-row mode and probably it can simplify code.

Regards

Pavel

2013/4/23 Christopher Manning <c(at)christophermanning(dot)org>

> psql currently collects the query result rows in memory before writing
> them to a file and can cause out of memory problems for large results in
> low memory environments like ec2. I can't use COPY TO STDOUT or FETCH_COUNT
> since I'm using Redshift and it doesn't support [writing to STDOUT](
> http://docs.aws.amazon.com/redshift/latest/dg/r_COPY.html) or [CURSOR](
> https://forums.aws.amazon.com/thread.jspa?threadID=122664).
>
> [Single Row Mode is available in Postgres 9.2](
> http://www.postgresql.org/docs/9.2/static/libpq-single-row-mode.html) but
> [it doesn't look like](
> http://www.postgresql.org/docs/9.2/static/app-psql.html) you can tell
> psql to use single row mode when writing to a file (using --output).
>
> I'm proposing to add a --single-row option to psql that would allow the
> result rows of a query to be streamed to a file without collecting them in
> memory first.
>
> I'm new to the postgres source, but I was considering doing this by adding
> an elseif at [this line in bin/psql/common.c](
> https://github.com/postgres/postgres/blob/master/src/bin/psql/common.c#L955) that
> would call [PQsetSingleRowMode](
> https://github.com/postgres/postgres/blob/master/src/interfaces/libpq/fe-exec.c#L1581) and
> ideally use something very similar to [ExecQueryUsingCursor](
> https://github.com/postgres/postgres/blob/master/src/bin/psql/common.c#L1081
> )
>
> Please let me know if that would be an acceptable addition and if there's
> anything in particular I should be aware of when adding the feature.
>
> Thank you,
> Christopher
>
>