On 04/10/2018 04:03 PM, Robert Haas wrote: > I suspect you want, or maybe need, to use the same snapshot as the > scan that retrieved the tuple containing the toasted datum.
I'm sure it's worth more than that, but I don't know if it's implementable. If I'm a function, and the datum came to me as a parameter, I may have no way to determine what snapshot the enclosing query used to obtain the thing passed to me. Or, if I found it myself, say by an SPI query within the function, usually that's at a level of abstraction somewhere above what-snapshot-was-used-in-the-scan. But in both cases, it's expected that I could successfully detoast either datum if I did so right there on the spot, as that's the usual convention, right? So at that moment, something in the set of registered || active snapshots is protecting the tuples I need. If it's impractical to determine which snapshot is needed (or just enough work to obviate any benefit of lazy detoasting), I wonder if there's at least a cheap way to check a particular snapshot for suitability wrt a given toast pointer. Check a couple usual suspects, find one most of the time, fall back to eager detoasting otherwise? Guess I need to go back for a deeper dive on just what constitutes a toast pointer. I was skimming last time.... -Chap
