opt: use optimizer_span_limit in split-scan rules

Use the optimizer_span_limit session setting as an additional upper
bound in splitScanIntoUnionScansOrSelects. When set, the span limit
caps the hard maximum scan count, preventing the split-scan rules
(SplitLimitedScanIntoUnionScans, SplitLimitedSelectIntoUnionSelects,
SplitGroupByScanIntoUnionScans, etc.) from generating more union
branches than the limit allows.

Release note: None

Author: michae2

pkg/sql/opt/xform/general_funcs.go

@@ -268,6 +268,9 @@ func (c *CustomFuncs) splitScanIntoUnionScansOrSelects(
 	// If OptSplitScanLimit is below maxScanCount, we will decrease maxScanCount
 	// to that value because a hard limit should never be lower than a soft limit.
 	hardMaxScanCount := int(c.e.evalCtx.SessionData().OptSplitScanLimit)
+	if spanLimit := int(c.e.evalCtx.SessionData().OptimizerSpanLimit); spanLimit > 0 && spanLimit < hardMaxScanCount {
+		hardMaxScanCount = spanLimit
+	}
 	if hardMaxScanCount < maxScanCount {
 		maxScanCount = hardMaxScanCount
 	}