Re: statement_timeout affects query results fetching?

Thanks for the explanation Robert, that makes total sense. However, it
seems like the utility of PG's statement_timeout is much more limited than
I thought.

In case you're interested, I dug a little further and it seems that
Microsoft's client for SQL Server implements the following timeout (source
<https://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.commandtimeout%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396>
):

cumulative time-out (for all network packets that are read during the
invocation of a method) for all network reads during command execution or
processing of the results. A time-out can still occur after the first row
is returned, and does not include user processing time, only network read
time.

Since it doesn't seem possible to have a clean query-processing-only
timeout at the backend, we may be better off doing something similar to the
above and enforce timeouts on the client only. Any further thoughts on this
would be appreciated.

On Sun, Aug 9, 2015 at 5:21 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Sat, Aug 8, 2015 at 11:30 AM, Shay Rojansky <roji(at)roji(dot)org> wrote:
> > the entire row in memory (imagine rows with megabyte-sized columns). This
> > makes sense to me; Tom, doesn't the libpq behavior you describe of
> absorbing
> > the result set as fast as possible mean that a lot of memory is wasted on
> > the client side?
>
> It sure does.
>
> > I can definitely appreciate the complexity of changing this behavior. I
> > think that some usage cases (such as mine) would benefit from a timeout
> on
> > the time until the first row is sent, this would allow to put an upper
> cap
> > on stuff like query complexity, for example.
>
> Unfortunately, it would not do any such thing. It's possible for the
> first row to be returned really really fast and then for an arbitrary
> amount of time to pass and computation to happen before all the rows
> are returned. A plan can have a startup cost of zero and a run cost
> of a billion (or a trillion). This kind of scenario isn't even
> particularly uncommon. You just need a plan that looks like this:
>
> Nested Loop
> -> Nested Loop
> -> Nested Loop
> -> Seq Scan
> -> Index Scan
> -> Index Scan
> -> Index Scan
>
> You can just keep pushing more nested loop/index scans on there and
> the first row will still pop out quite fast. But if the seq-scanned
> table is large, generating the whole result set can take a long, long
> time.
>
> Even worse, you could have a case like this:
>
> SELECT somefunc(a) FROM foo;
>
> Now suppose somefunc is normally very quick, but if a = 42 then it
> does pg_sleep() in a loop until the world ends. You're going to have
> however many rows of foo have a != 42 pop out near-instantaneously,
> and then it will go into the tank and not come out until the meaning
> of life, the universe, and everything is finally revealed.
>
> That second case is a little extreme, and a little artificial, but the
> point is this: just as you don't want the client to have to buffer the
> results in memory, the server doesn't either. It's not the case that
> the server computes the rows and sends them to you. Each one is sent
> to you as it is computed, and in the normal case, at the time the
> first row is sent, only a small percentage of the total work of the
> query has been performed.
>
> --
> Robert Haas
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>