Improve ANALYZE's handling of concurrent-update scenarios.

This patch changes the rule for whether or not a tuple seen by ANALYZE
should be included in its sample.

When we last touched this logic, in commit 51e1445, we weren't
thinking very hard about tuples being UPDATEd by a long-running
concurrent transaction.  In such a case, we might see the pre-image as
either LIVE or DELETE_IN_PROGRESS depending on timing; and we might see
the post-image not at all, or as INSERT_IN_PROGRESS.  Since the existing
code will not sample either DELETE_IN_PROGRESS or INSERT_IN_PROGRESS
tuples, this leads to concurrently-updated rows being omitted from the
sample entirely.  That's not very helpful, and it's especially the wrong
thing if the concurrent transaction ends up rolling back.

The right thing seems to be to sample DELETE_IN_PROGRESS rows just as if
they were live.  This makes the "sample it" and "count it" decisions the
same, which seems good for consistency.  It's clearly the right thing
if the concurrent transaction ends up rolling back; in effect, we are
sampling as though IN_PROGRESS transactions haven't happened yet.
Also, this combination of choices ensures maximum robustness against
the different combinations of whether and in which state we might see the
pre- and post-images of an update.

It's slightly annoying that we end up recording immediately-out-of-date
stats in the case where the transaction does commit, but on the other
hand the stats are fine for columns that didn't change in the update.
And the alternative of sampling INSERT_IN_PROGRESS rows instead seems
like a bad idea, because then the sampling would be inconsistent with
the way rows are counted for the stats report.

Per report from Mark Chambers; thanks to Jeff Janes for diagnosing
what was happening.  Back-patch to all supported versions.

Discussion: https://postgr.es/m/CAFh58O_Myr6G3tcH3gcGrF-=OExB08PJdWZcSBcEcovaiPsrHA@mail.gmail.com

Author: tglsfdc

src/backend/commands/analyze.c

@@ -1124,20 +1124,35 @@ acquire_sample_rows(Relation onerel, int elevel,
 				case HEAPTUPLE_DELETE_IN_PROGRESS:
 
 					/*
-					 * We count delete-in-progress rows as still live, using
-					 * the same reasoning given above; but we don't bother to
-					 * include them in the sample.
+					 * We count and sample delete-in-progress rows the same as
+					 * live ones, so that the stats counters come out right if
+					 * the deleting transaction commits after us, per the same
+					 * reasoning given above.
 					 *
 					 * If the delete was done by our own transaction, however,
 					 * we must count the row as dead to make
 					 * pgstat_report_analyze's stats adjustments come out
 					 * right.  (Note: this works out properly when the row was
 					 * both inserted and deleted in our xact.)
+					 *
+					 * The net effect of these choices is that we act as
+					 * though an IN_PROGRESS transaction hasn't happened yet,
+					 * except if it is our own transaction, which we assume
+					 * has happened.
+					 *
+					 * This approach ensures that we behave sanely if we see
+					 * both the pre-image and post-image rows for a row being
+					 * updated by a concurrent transaction: we will sample the
+					 * pre-image but not the post-image.  We also get sane
+					 * results if the concurrent transaction never commits.
 					 */
 					if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetUpdateXid(targtuple.t_data)))
 						deadrows += 1;
 					else
+					{
+						sample_it = true;
 						liverows += 1;
+					}
 					break;
 
 				default: