rowexec: fix usage of the shared "single datum" acc in windower

[yuzefovich]

This commit fixes the usage of the shared "single datum agg mem account" by the window builtins. Previously, we were updating the eval context with the memory account after the builtins were constructed. The impact of the bug on its own is pretty minor (namely that we could reserve more memory than necessary - the initial allocation is 10KiB), but I have some other changes brewing which make it more important to be precise about the attribution of the memory allocations.

Release note: None

[cockroach-teamcity]

This change is 

[DrewKimball]

So, just to make sure I understand, the problem is that not setting the account causes a monitor to be created for each aggregate, which leads to an overestimate because we amortize the cost of accounting by reserving more than the exact amount requested. Is that right?

Reviewable status: complete! 1 of 0 LGTMs obtained

[yuzefovich]

Yes, pretty much. The only minor difference is that an account (not a monitor) is created for each aggregate builtin. Then, regardless of the memory usage of that particular builtin, once that separate account is grown, its initial allocation will be at least 10KiB. This was very problematic for the row-by-row hash aggregator which creates a separate builtin object for each bucket (i.e. each group), but for window functions we reuse the same builtin objects for each row. Still, if we have a query with 100 window functions, then we might reserve 1MiB from the monitor and only use a small fraction of that.

There is some more historical context in this PR description as well as this comment.

TFTR!

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

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