Subject: Add a property to automatically suspend portals as they produce given number of bytes

From: Vladimir Sitnikov <[email-masked]>
Date: Fri, 17 Jan 2025 14:08:15 +0300

Hi,

Applications often face an "out of memory" condition as they try to fetch
"N rows" from the database.
If N is small, then the execution becomes inefficient due to many
roundtrips.
If N is high, there's a risk that many rows would overflow the client's
memory.

Note: the client can't stop reading the data at arbitrary times as they
might want to execute another query
while processing the resultset, so "fetch all rows at once and read them
from the socket as needed" is not really an option.

The problem is the clients can't tell how much memory it would take to
buffer the results of "fetch 100 rows".
The row size might vary.

Currently, the only case the database returns PortalSuspended seems to be
when max_rows is reached when executing a select.
I wonder if the database can suspend portals earlier in case it knows it
produced a lot of data.

In other words:
1) If the user calls execute(fetch all rows), then follow the current
behavior.
2) If the user provides non-zero max_rows, then optionally suspend the
portal as the result exceeds a pre-configured amount (GUC?).

Then clients could configure the connection like "ok, please suspend
portals if a single fetch exceeds 50MiB".
To my understanding,
* it should be a backward-compatible change
* it would require no protocol changes,
* it would make applications more robust when it comes to "out of memory
while fetching many rows"
* it might improve the performance overall as the applications could
increase their pessimistic "max_rows" fetch size

Is it ok to contribute a patch like that?

Vladimir