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Fix false positive parallel skew warning at DOP 2 #74

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erikdarlingdata merged 1 commit into from
Mar 10, 2026
+8 −4
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Fix false positive parallel skew warning at DOP 2 #74

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12 changes: 8 additions & 4 deletions src/PlanViewer.Core/Services/PlanAnalyzer.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -600,15 +600,19 @@

// Rule 8: Parallel thread skew (actual plans with per-thread stats)
// Only warn when there are enough rows to meaningfully distribute across threads
// Filter out thread 0 (coordinator) which typically does 0 rows in parallel operators
if (!cfg.IsRuleDisabled(8) && node.PerThreadStats.Count > 1)
{
var totalRows = node.PerThreadStats.Sum(t => t.ActualRows);
var minRowsForSkew = node.PerThreadStats.Count * 1000;
var workerThreads = node.PerThreadStats.Where(t => t.ThreadId > 0).ToList();
if (workerThreads.Count < 2) workerThreads = node.PerThreadStats; // fallback
var totalRows = workerThreads.Sum(t => t.ActualRows);
var minRowsForSkew = workerThreads.Count * 1000;
if (totalRows >= minRowsForSkew)
{
var maxThread = node.PerThreadStats.OrderByDescending(t => t.ActualRows).First();
var maxThread = workerThreads.OrderByDescending(t => t.ActualRows).First();
var skewRatio = (double)maxThread.ActualRows / totalRows;
var skewThreshold = node.PerThreadStats.Count == 2 ? 0.75 : 0.50;
// At DOP 2, a 60/40 split is normal — use higher threshold
var skewThreshold = workerThreads.Count <= 2 ? 0.80 : 0.50;
if (skewRatio >= skewThreshold)
{
var message = $"Thread {maxThread.ThreadId} processed {skewRatio:P0} of rows ({maxThread.ActualRows:N0}/{totalRows:N0}). Work is heavily skewed to one thread, so parallelism isn't helping much.";
Expand Down Expand Up @@ -939,7 +943,7 @@
// Rule 28: Row Count Spool — NOT IN with nullable column
// Pattern: Row Count Spool with high rewinds, child scan has IS NULL predicate,
// and statement text contains NOT IN
if (!cfg.IsRuleDisabled(28) && node.PhysicalOp.Contains("Row Count Spool"))

Check warning on line 946 in src/PlanViewer.Core/Services/PlanAnalyzer.cs

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Dereference of a possibly null reference.

Check warning on line 946 in src/PlanViewer.Core/Services/PlanAnalyzer.cs

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GitHub Actions / build-and-test 

Dereference of a possibly null reference.
{
var rewinds = node.HasActualStats ? (double)node.ActualRewinds : node.EstimateRewinds;
if (rewinds > 10000 && HasNotInPattern(node, stmt))
Expand Down
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