Hi, On 2019-02-25 11:59:06 -0800, Peter Geoghegan wrote: > On Mon, Feb 25, 2019 at 10:59 AM Tom Lane <t...@sss.pgh.pa.us> wrote: > > Because it involves touching ten times more code (and that's a very > > conservative estimate). Excluding changes in pg_list.h + list.c, > > what I posted touches approximately 600 lines of code (520 insertions, > > 644 deletions to be exact). For comparison's sake, there are about > > 1800 uses of foreach in the tree, each of which would require at least > > 3 changes to replace (the foreach itself, the ListCell variable > > declaration, and at least one lfirst() reference in the loop body). > > If we knew that the list code was the bottleneck in a handful of > cases, then I'd come down on Robert's side here. It would then be > possible to update the relevant bottlenecked code in an isolated > fashion, while getting the lion's share of the benefit. However, I > don't think that that's actually possible. The costs of using Lists > everywhere are real and measurable, but it's also highly distributed. > At least, that's my recollection from previous discussion from several > years back. I remember talking about this with Andres in early 2016.
It's distributed, but not *that* distributed. The largest source of "cost" at execution time used to be all-over expression evaluation, but that's gone now. That was a lot of places, but it's not outside of reach of a targeted change. Now it's targetlist handling, which'd have to change together with plan time, where it's a large issue. > > So we've already blown past 5000 lines worth of changes if we want to > > do it another way ... and that's just *one* component of the List API. > > If you want to stop third party code from compiling, you can find a > way to do that without really changing your approach. Nothing stops > you from changing some symbol names minimally, and then making > corresponding mechanical changes to all of the client code within the > tree. Third party code authors would then follow this example, with > the expectation that it's probably going to be a totally mechanical > process. > > I'm not necessarily advocating that approach. I'm simply pointing out > that a compromise is quite possible. That breaks extension code using lists unnecessarily though. And given that there's semantic change, I don't htink it's an entirely mechanical process. Greetings, Andres Freund
