Re: Strange slow behavior in backend

Tom Lane wrote:
> Kyle <kyle(at)actarg(dot)com> writes:
> > This worked great until I put a real big file in (about 5M). Then, when
> > I tried to fetch the file, it seemed really slow (about 60 seconds). I
> > tried reassembling the file in the frontend instead and my time dropped
> > to about 6 seconds using this TCL fragment (mpg::qlist is an interface
> > to pg_exec that returns a list of tuples):
> > The only difference I can identify is whether the re-assembly TCL code
> > is running as a procedural language (backend) or in the frontend.
> > Anyone have any idea why the difference is so dramatic?
>
> I happened to have handy a 7.1 backend compiled for profiling, so I
> looked into this a little. I confirm that this seems unreasonably slow.
> As near as I can tell, 98% of the backend runtime is being spent in
> strlen() and strcpy() invoked from Tcl_SetResult invoked from Tcl_Eval
> invoked from the per-result-tuple loop in pltcl_SPI_exec. Apparently,
> all this is happening because Tcl_Eval thinks it needs to make the
> result of the append command available for its caller. I changed the
> inner loop to
>
> spi_exec -array d "select data from pg_largeobject where
> loid = $1 order by pageno" {
> append odata $d(data);
> set z z
> }
>
> and voila, the runtime dropped to something reasonable.
>
> So, yes, it would seem that some care in the inner loop of
> pltcl_SPI_exec would help a lot. It'd be worth if'defing the Tcl_Eval
> call there to use a new-style call when using Tcl 8. (This could also
> avoid repetitive parsing of the loop body.) Might want to think about
> the same for the Tcl function as a whole, too.
>
> I was also distressed to notice that pltcl_set_tuple_values does a LOT
> of repetitive work --- it should be fixed so that the syscache and
> function lookups are done only once, not once per tuple.

Hmmm - seems worse than I thought.

Again, let's wait with all that until we can overhaul it
including the tupleset return mechanism and simply loose
pre-8.0 compatibility.

Jan

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