Re: ECPG FETCH readahead

On Fri, Mar 02, 2012 at 11:41:05AM -0500, Noah Misch wrote:
> We yet lack a consensus on whether native ECPG apps should have access to the
> feature. My 2c is to make it available, because it's useful syntactic sugar.
> If your program independently processes each row of an arbitrary-length result
> set, current facilities force you to add an extra outer loop to batch the
> FETCHes for every such code site. Applications could define macros to
> abstract that pattern, but this seems common-enough to justify bespoke
> handling. Besides, minimalists already use libpq directly.

Sorry, I don't really understand what you're saying here. The program logic
won't change at all when using this feature or what do I misunderstand?

> I suggest enabling the feature by default but drastically reducing the default
> readahead chunk size from 256 to, say, 5. That still reduces the FETCH round
> trip overhead by 80%, but it's small enough not to attract pathological
> behavior on a workload where each row is a 10 MiB document. I would not offer
> an ecpg-time option to disable the feature per se. Instead, let the user set
> the default chunk size at ecpg time. A setting of 1 effectively disables the
> feature, though one could later re-enable it with ECPGFETCHSZ.

Using 1 to effectively disable the feature is fine with me, but I strongly
object any default enabling this feature. It's farily easy to create cases with
pathological behaviour and this features is not standard by any means. I figure
a normal programmer would expect only one row being transfered when fetching
one.

Other than that, thanks for the great review.

Michael
--
Michael Meskes
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