Re: TransactionIdIsInProgress() cache

I wrote:
> [ after reading the patch a bit closer... ] Actually, it's not as
> obvious as all that. TransactionIdIsInProgress already falls out before
> the proposed test for any XID < RecentXmin, which means that in reality
> it's fairly probable that the target XID is running. So while this test
> may not cost much, it's not clear that it really buys much either.

I realized that a pretty simple test was already built into the code:
look at the stats gathered by XIDCACHE_DEBUG. Here are the totals for
a set of parallel regression tests:

xmin: 185668
known: 25728
myxact: 5521
latest: 239
mainxid: 484
childxid: 0
nooflo: 3331
slow: 0

So even though the xmin test takes out a lot, this seems worth doing.
Some more-formal performance testing might still be a good idea though.

For completeness, the raw per-backend stats are attached.
Unsurprisingly, the wins come in bunches.

regards, tom lane

XidCache: xmin: 1, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 1, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 0, known: 0, myxact: 4, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 0, known: 9, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 0, known: 14, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 17, slow: 0
XidCache: xmin: 1, known: 3, myxact: 1, latest: 0, mainxid: 0, childxid: 0, nooflo: 3, slow: 0
XidCache: xmin: 0, known: 11, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 11, slow: 0
XidCache: xmin: 18, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 0, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 0, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 0, known: 2, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 0, known: 6, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 8, slow: 0
XidCache: xmin: 32, known: 4, myxact: 2, latest: 0, mainxid: 0, childxid: 0, nooflo: 5, slow: 0
XidCache: xmin: 0, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 15, slow: 0
XidCache: xmin: 25, known: 2, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 2, slow: 0
XidCache: xmin: 6, known: 5, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 13, slow: 0
XidCache: xmin: 103, known: 2, myxact: 0, latest: 2, mainxid: 0, childxid: 0, nooflo: 5, slow: 0
XidCache: xmin: 174, known: 0, myxact: 13, latest: 0, mainxid: 1, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 94, known: 54, myxact: 1, latest: 2, mainxid: 0, childxid: 0, nooflo: 28, slow: 0
XidCache: xmin: 97, known: 89, myxact: 19, latest: 0, mainxid: 2, childxid: 0, nooflo: 50, slow: 0
XidCache: xmin: 2401, known: 12, myxact: 17, latest: 2, mainxid: 0, childxid: 0, nooflo: 16, slow: 0
XidCache: xmin: 156, known: 11, myxact: 15, latest: 0, mainxid: 0, childxid: 0, nooflo: 5, slow: 0
XidCache: xmin: 137, known: 6, myxact: 9, latest: 0, mainxid: 0, childxid: 0, nooflo: 4, slow: 0
XidCache: xmin: 8, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 6, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 59, known: 3, myxact: 5, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 7, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 5, slow: 0
XidCache: xmin: 0, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 43, known: 1, myxact: 7, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 1, known: 3, myxact: 1, latest: 0, mainxid: 0, childxid: 0, nooflo: 5, slow: 0
XidCache: xmin: 1, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 0, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 8, slow: 0
XidCache: xmin: 0, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 14, slow: 0
XidCache: xmin: 0, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 6, known: 4, myxact: 1, latest: 0, mainxid: 0, childxid: 0, nooflo: 19, slow: 0
XidCache: xmin: 0, known: 3, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 11, slow: 0
XidCache: xmin: 12, known: 8, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 14, slow: 0
XidCache: xmin: 0, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 42, known: 7, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 35, slow: 0
XidCache: xmin: 15, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 2, slow: 0
XidCache: xmin: 308, known: 8, myxact: 67, latest: 0, mainxid: 0, childxid: 0, nooflo: 2, slow: 0
XidCache: xmin: 51, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 33, slow: 0
XidCache: xmin: 54, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 27, known: 1, myxact: 1, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 62, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 120, known: 7, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 79, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 38, known: 8, myxact: 15, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 34, known: 0, myxact: 3, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 81, known: 2, myxact: 1, latest: 0, mainxid: 0, childxid: 0, nooflo: 3, slow: 0
XidCache: xmin: 344, known: 0, myxact: 42, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 17, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 139, known: 12, myxact: 24, latest: 0, mainxid: 0, childxid: 0, nooflo: 3, slow: 0
XidCache: xmin: 0, known: 1, myxact: 0, latest: 2, mainxid: 0, childxid: 0, nooflo: 10, slow: 0
XidCache: xmin: 1220, known: 5, myxact: 31, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 1, known: 0, myxact: 6, latest: 4, mainxid: 0, childxid: 0, nooflo: 5, slow: 0
XidCache: xmin: 19, known: 8189, myxact: 0, latest: 1, mainxid: 16, childxid: 0, nooflo: 57, slow: 0
XidCache: xmin: 0, known: 69, myxact: 8, latest: 2, mainxid: 2, childxid: 0, nooflo: 23, slow: 0
XidCache: xmin: 443, known: 291, myxact: 122, latest: 6, mainxid: 17, childxid: 0, nooflo: 87, slow: 0
XidCache: xmin: 408, known: 195, myxact: 89, latest: 11, mainxid: 27, childxid: 0, nooflo: 140, slow: 0
XidCache: xmin: 20655, known: 498, myxact: 2, latest: 4, mainxid: 2, childxid: 0, nooflo: 5, slow: 0
XidCache: xmin: 1545, known: 197, myxact: 252, latest: 16, mainxid: 103, childxid: 0, nooflo: 82, slow: 0
XidCache: xmin: 780, known: 220, myxact: 330, latest: 2, mainxid: 4, childxid: 0, nooflo: 76, slow: 0
XidCache: xmin: 0, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 103040, known: 15, myxact: 32, latest: 0, mainxid: 0, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 4028, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 0, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 55, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 2, slow: 0
XidCache: xmin: 1000, known: 4, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 61, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 0, known: 43, myxact: 8, latest: 0, mainxid: 0, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 21, known: 4, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 0, known: 11, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 6, slow: 0
XidCache: xmin: 36, known: 20, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 5, slow: 0
XidCache: xmin: 0, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 32, known: 22, myxact: 6, latest: 0, mainxid: 0, childxid: 0, nooflo: 9, slow: 0
XidCache: xmin: 3, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 3, slow: 0
XidCache: xmin: 1, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 35, known: 17, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 4, slow: 0
XidCache: xmin: 13, known: 74, myxact: 18, latest: 0, mainxid: 8, childxid: 0, nooflo: 17, slow: 0
XidCache: xmin: 2, known: 4, myxact: 2, latest: 0, mainxid: 0, childxid: 0, nooflo: 5, slow: 0
XidCache: xmin: 26, known: 14, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 5, slow: 0
XidCache: xmin: 12, known: 14, myxact: 1, latest: 7, mainxid: 13, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 111, known: 63, myxact: 10, latest: 2, mainxid: 5, childxid: 0, nooflo: 60, slow: 0
XidCache: xmin: 1, known: 106, myxact: 16, latest: 0, mainxid: 0, childxid: 0, nooflo: 31, slow: 0
XidCache: xmin: 30, known: 0, myxact: 0, latest: 0, mainxid: 1, childxid: 0, nooflo: 2, slow: 0
XidCache: xmin: 191, known: 106, myxact: 26, latest: 0, mainxid: 0, childxid: 0, nooflo: 58, slow: 0
XidCache: xmin: 257, known: 19, myxact: 44, latest: 0, mainxid: 0, childxid: 0, nooflo: 10, slow: 0
XidCache: xmin: 10519, known: 59, myxact: 19, latest: 0, mainxid: 1, childxid: 0, nooflo: 53, slow: 0
XidCache: xmin: 2, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 29, known: 1, myxact: 2, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 119, known: 14, myxact: 13, latest: 0, mainxid: 0, childxid: 0, nooflo: 6, slow: 0
XidCache: xmin: 23, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 151, known: 0, myxact: 23, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 6392, known: 9, myxact: 69, latest: 0, mainxid: 0, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 1940, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 17, known: 19, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 0, known: 173, myxact: 62, latest: 0, mainxid: 11, childxid: 0, nooflo: 12, slow: 0
XidCache: xmin: 92, known: 23, myxact: 11, latest: 0, mainxid: 10, childxid: 0, nooflo: 14, slow: 0
XidCache: xmin: 176, known: 252, myxact: 97, latest: 3, mainxid: 19, childxid: 0, nooflo: 88, slow: 0
XidCache: xmin: 455, known: 133, myxact: 199, latest: 11, mainxid: 8, childxid: 0, nooflo: 104, slow: 0
XidCache: xmin: 45, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 22, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 15, known: 31, myxact: 4, latest: 0, mainxid: 0, childxid: 0, nooflo: 12, slow: 0
XidCache: xmin: 1415, known: 468, myxact: 408, latest: 14, mainxid: 20, childxid: 0, nooflo: 325, slow: 0
XidCache: xmin: 3568, known: 631, myxact: 1507, latest: 38, mainxid: 21, childxid: 0, nooflo: 231, slow: 0
XidCache: xmin: 3, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 62, known: 51, myxact: 7, latest: 1, mainxid: 0, childxid: 0, nooflo: 27, slow: 0
XidCache: xmin: 89, known: 5, myxact: 5, latest: 2, mainxid: 19, childxid: 0, nooflo: 7, slow: 0
XidCache: xmin: 35, known: 228, myxact: 44, latest: 4, mainxid: 2, childxid: 0, nooflo: 48, slow: 0
XidCache: xmin: 3, known: 4, myxact: 0, latest: 0, mainxid: 2, childxid: 0, nooflo: 9, slow: 0
XidCache: xmin: 61, known: 32, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 2, slow: 0
XidCache: xmin: 84, known: 23, myxact: 2, latest: 2, mainxid: 6, childxid: 0, nooflo: 69, slow: 0
XidCache: xmin: 182, known: 324, myxact: 46, latest: 2, mainxid: 2, childxid: 0, nooflo: 27, slow: 0
XidCache: xmin: 60, known: 97, myxact: 31, latest: 0, mainxid: 2, childxid: 0, nooflo: 24, slow: 0
XidCache: xmin: 66, known: 187, myxact: 52, latest: 0, mainxid: 29, childxid: 0, nooflo: 38, slow: 0
XidCache: xmin: 114, known: 163, myxact: 209, latest: 0, mainxid: 0, childxid: 0, nooflo: 50, slow: 0
XidCache: xmin: 178, known: 96, myxact: 147, latest: 10, mainxid: 27, childxid: 0, nooflo: 45, slow: 0
XidCache: xmin: 107, known: 31, myxact: 23, latest: 4, mainxid: 8, childxid: 0, nooflo: 20, slow: 0
XidCache: xmin: 8084, known: 10445, myxact: 0, latest: 13, mainxid: 30, childxid: 0, nooflo: 80, slow: 0
XidCache: xmin: 128, known: 205, myxact: 74, latest: 6, mainxid: 18, childxid: 0, nooflo: 155, slow: 0
XidCache: xmin: 670, known: 14, myxact: 8, latest: 0, mainxid: 4, childxid: 0, nooflo: 10, slow: 0
XidCache: xmin: 290, known: 286, myxact: 72, latest: 2, mainxid: 15, childxid: 0, nooflo: 155, slow: 0
XidCache: xmin: 2231, known: 4, myxact: 198, latest: 8, mainxid: 0, childxid: 0, nooflo: 2, slow: 0
XidCache: xmin: 3142, known: 1040, myxact: 741, latest: 48, mainxid: 20, childxid: 0, nooflo: 373, slow: 0
XidCache: xmin: 0, known: 0, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 1225, known: 166, myxact: 122, latest: 8, mainxid: 9, childxid: 0, nooflo: 214, slow: 0
XidCache: xmin: 157, known: 0, myxact: 7, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0
XidCache: xmin: 125, known: 1, myxact: 0, latest: 0, mainxid: 0, childxid: 0, nooflo: 1, slow: 0
XidCache: xmin: 4766, known: 0, myxact: 68, latest: 0, mainxid: 0, childxid: 0, nooflo: 0, slow: 0