sql/stats: bring back guard against non-zero NumRange in forecasts

It occurred to me tonight that the code we removed in #143955 not only
generated a sentry report, but also returned an error instead of
producing a faulty histogram. I too hope that #93892 is now fixed, but
it seems wise to at least keep some code that guards against faulty
histograms, even if we don't think the sentry report is necessary any
more.

Informs: #93892

Epic: None

Release note: None

Author: michae2

pkg/sql/stats/forecast.go

@@ -16,7 +16,9 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/settings/cluster"
 	"github.com/cockroachdb/cockroach/pkg/sql/opt/cat"
 	"github.com/cockroachdb/cockroach/pkg/sql/sem/eval"
+	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
 	"github.com/cockroachdb/cockroach/pkg/sql/types"
+	"github.com/cockroachdb/cockroach/pkg/util/errorutil"
 	"github.com/cockroachdb/cockroach/pkg/util/log"
 	"github.com/cockroachdb/errors"
 	"github.com/cockroachdb/redact"
@@ -329,6 +331,24 @@ func forecastColumnStatistics(
 		}
 		forecast.HistogramData = &histData
 		forecast.setHistogramBuckets(hist)
+
+		// Verify that the first two buckets (the initial NULL bucket and the first
+		// non-NULL bucket) both have NumRange=0 and DistinctRange=0. (We must check
+		// this after calling setHistogramBuckets to account for rounding.)
+		for _, bucket := range forecast.Histogram {
+			if bucket.NumRange != 0 || bucket.DistinctRange != 0 {
+				err := errorutil.UnexpectedWithIssueErrorf(
+					93892, "forecasted histogram for table %v had first bucket with non-zero NumRange or "+
+						"DistinctRange", tableID,
+				)
+				log.Warningf(ctx, "%v", err)
+				return nil, err
+			}
+			if bucket.UpperBound != tree.DNull {
+				// Stop checking after the first non-NULL bucket.
+				break
+			}
+		}
 	}
 
 	return forecast, nil