Re: Using Expanded Objects other than Arrays from plpgsql - Mailing list pgsql-hackers
From | Tom Lane |
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Subject | Re: Using Expanded Objects other than Arrays from plpgsql |
Date | |
Msg-id | 2265411.1729699470@sss.pgh.pa.us Whole thread Raw |
In response to | Re: Using Expanded Objects other than Arrays from plpgsql (Tom Lane <tgl@sss.pgh.pa.us>) |
Responses |
Re: Using Expanded Objects other than Arrays from plpgsql
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List | pgsql-hackers |
I wrote: > One idea I was toying with is that it doesn't matter if f() > throws an error so long as the plpgsql function is not executing > within an exception block: if the error propagates out of the plpgsql > function then we no longer care about the value of the variable. > That would very substantially weaken the requirements on how f() > is implemented. The more I think about this idea the better I like it. We can improve on the original concept a bit: the assignment can be within an exception block so long as the target variable is too. For example, consider DECLARE x float8[]; BEGIN ... DECLARE y float8[]; BEGIN x := array_append(x, 42); y := array_append(y, 42); END; EXCEPTION WHEN ...; END; Currently, both calls of array_append are subject to R/W optimization, so that array_append must provide a strong guarantee that it won't throw an error after it's begun to change the R/W object. If we redefine things so that the optimization is applied only to "y", then AFAICS we need nothing from array_append. It only has to be sure it doesn't corrupt the object so badly that it can't be freed ... but that requirement exists already, for anything dealing with expanded objects. So this would put us in a situation where we could apply the optimization by default, which'd be a huge win. There is an exception: if we are considering x := array_cat(x, x); then I don't think we can optimize because of the aliasing problem I mentioned before. So there'd have to be a restriction that the target variable is mentioned only once in the function's arguments. For stuff like your vxm() function, that'd be annoying. But functions that need that and are willing to deal with the aliasing hazard could still provide a prosupport function that promises it's okay. What we'd accomplish is that a large fraction of interesting functions could get the benefit without having to create a prosupport function, which is a win all around. Also worth noting: in the above example, we could optimize the update on "x" too, if we know that "x" is not referenced in the block's EXCEPTION handlers. I wouldn't bother with this in the first version, but it might be worth doing later. So if we go this way, the universe of functions that can benefit from the optimization enlarges considerably, and the risk of bugs that break the optimization drops considerably. The cost is that some cases that were optimized before now will not be. But I suspect that plpgsql functions where this optimization is key probably don't contain EXCEPTION handlers at all, so that they won't notice any change. Thoughts? regards, tom lane
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