Re: out of memory in crosstab()

On 11/16/22 02:47, Amit Langote wrote:
> A customer seems to have run into $subject. Here's a reproducer they shared:

> With the following logged:
>
> LOG: server process (PID 121846) was terminated by signal 9: Killed

That's the Linux OOM killer. Was this running in a container or under
systemd with memory.limit_in_bytes set? If so, perhaps they need a
higher setting.

> The problem seems to be spi_printtup() continuing to allocate memory
> to expand _SPI_current->tuptable to store the result of crosstab()'s
> input query that's executed using:
>
> /* Retrieve the desired rows */
> ret = SPI_execute(sql, true, 0);
>
> Note that this asks SPI to retrieve and store *all* result rows of the
> query in _SPI_current->tuptable, and if there happen to be so many
> rows, as in the case of above example, spi_printtup() ends up asking
> for a bit too much memory.

check

> The easiest fix for this seems to be for crosstab() to use open a
> cursor (SPI_cursor_open) and fetch the rows in batches
> (SPI_cursor_fetch) rather than all in one go. I have implemented that
> in the attached. Maybe the patch should address other functions that
> potentially have the same problem.

Seems reasonable. I didn't look that closely at the patch, but I do
think that there needs to be some justification for the selected batch
size and/or make it configurable.

> I also wondered about fixing this by making _SPI_current->tuptable use
> a tuplestore that can spill to disk as its backing store rather than a
> plain C HeapTuple array, but haven't checked how big of a change that
> would be; SPI_tuptable is referenced in many places across the tree.
> Though I suspect that idea has enough merits to give that a try
> someday.

Seems like a separate patch at the very least

> Thoughts on whether this should be fixed and the fix be back-patched?

-1 on backpatching -- this is not a bug, and the changes are non-trivial

Joe

--
Joe Conway
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com