compare-perf: Add support for selecting a single run as input

A very hacky implementation that simply instantiates an empty
`PerformanceOverviewScanner` as the "from" column (i.e. with all values
empty). To see it in action, select a single query run in the query
history and pick "Compare Performance" from the context menu. Then
select the "Single run" option when prompted.

extensions/ql-vscode/src/compare-performance/compare-performance-view.ts

@@ -56,8 +56,10 @@ export class ComparePerformanceView extends AbstractWebview<
     }
 
     const [fromPerf, toPerf] = await Promise.all([
-      scanLogWithProgress(fromJsonLog, "1/2"),
-      scanLogWithProgress(toJsonLog, "2/2"),
+      fromJsonLog === ""
+        ? new PerformanceOverviewScanner()
+        : scanLogWithProgress(fromJsonLog, "1/2"),
+      scanLogWithProgress(toJsonLog, fromJsonLog === "" ? "1/1" : "2/2"),
     ]);
 
     await this.postMessage({

extensions/ql-vscode/src/extension.ts

@@ -927,7 +927,7 @@ async function activateWithInstalledDistribution(
     ): Promise<void> => showResultsForComparison(compareView, from, to),
     async (
       from: CompletedLocalQueryInfo,
-      to: CompletedLocalQueryInfo,
+      to: CompletedLocalQueryInfo | undefined,
     ): Promise<void> =>
       showPerformanceComparison(comparePerformanceView, from, to),
   );
@@ -1208,17 +1208,22 @@ async function showResultsForComparison(
 async function showPerformanceComparison(
   view: ComparePerformanceView,
   from: CompletedLocalQueryInfo,
-  to: CompletedLocalQueryInfo,
+  to: CompletedLocalQueryInfo | undefined,
 ): Promise<void> {
-  const fromLog = from.evaluatorLogPaths?.jsonSummary;
-  const toLog = to.evaluatorLogPaths?.jsonSummary;
+  let fromLog = from.evaluatorLogPaths?.jsonSummary;
+  let toLog = to?.evaluatorLogPaths?.jsonSummary;
+
+  if (to === undefined) {
+    toLog = fromLog;
+    fromLog = "";
+  }
   if (fromLog === undefined || toLog === undefined) {
     return extLogger.showWarningMessage(
       `Cannot compare performance as the structured logs are missing. Did they queries complete normally?`,
     );
   }
   await extLogger.log(
-    `Comparing performance of ${from.getQueryName()} and ${to.getQueryName()}`,
+    `Comparing performance of ${from.getQueryName()} and ${to?.getQueryName() ?? "baseline"}`,
   );
 
   await view.showResults(fromLog, toLog);

extensions/ql-vscode/src/query-history/query-history-manager.ts

@@ -151,7 +151,7 @@ export class QueryHistoryManager extends DisposableObject {
     ) => Promise<void>,
     private readonly doComparePerformanceCallback: (
       from: CompletedLocalQueryInfo,
-      to: CompletedLocalQueryInfo,
+      to: CompletedLocalQueryInfo | undefined,
     ) => Promise<void>,
   ) {
     super();
@@ -706,7 +706,10 @@ export class QueryHistoryManager extends DisposableObject {
 
     let toItem: CompletedLocalQueryInfo | undefined = undefined;
     try {
-      toItem = await this.findOtherQueryToCompare(fromItem, multiSelect);
+      toItem = await this.findOtherQueryToComparePerformance(
+        fromItem,
+        multiSelect,
+      );
     } catch (e) {
       void showAndLogErrorMessage(
         this.app.logger,
@@ -714,10 +717,8 @@ export class QueryHistoryManager extends DisposableObject {
       );
     }
 
-    if (toItem !== undefined) {
     await this.doComparePerformanceCallback(fromItem, toItem);
   }
-  }
 
   async handleItemClicked(item: QueryHistoryInfo) {
     this.treeDataProvider.setCurrentItem(item);
@@ -1116,6 +1117,7 @@ export class QueryHistoryManager extends DisposableObject {
         detail: item.completedQuery.message,
         query: item,
       }));
+
     if (comparableQueryLabels.length < 1) {
       throw new Error("No other queries available to compare with.");
     }
@@ -1124,6 +1126,60 @@ export class QueryHistoryManager extends DisposableObject {
     return choice?.query;
   }
 
+  private async findOtherQueryToComparePerformance(
+    fromItem: CompletedLocalQueryInfo,
+    allSelectedItems: CompletedLocalQueryInfo[],
+  ): Promise<CompletedLocalQueryInfo | undefined> {
+    const dbName = fromItem.databaseName;
+
+    // If exactly 2 items are selected, return the one that
+    // isn't being used as the "from" item.
+    if (allSelectedItems.length === 2) {
+      const otherItem =
+        fromItem === allSelectedItems[0]
+          ? allSelectedItems[1]
+          : allSelectedItems[0];
+      if (otherItem.databaseName !== dbName) {
+        throw new Error("Query databases must be the same.");
+      }
+      return otherItem;
+    }
+
+    if (allSelectedItems.length > 2) {
+      throw new Error("Please select no more than 2 queries.");
+    }
+
+    // Otherwise, present a dialog so the user can choose the item they want to use.
+    const comparableQueryLabels = this.treeDataProvider.allHistory
+      .filter(this.isSuccessfulCompletedLocalQueryInfo)
+      .filter(
+        (otherItem) =>
+          otherItem !== fromItem && otherItem.databaseName === dbName,
+      )
+      .map((item) => ({
+        label: this.labelProvider.getLabel(item),
+        description: item.databaseName,
+        detail: item.completedQuery.message,
+        query: item,
+      }));
+    const comparableQueryLabelsWithDefault = [
+      {
+        label: "Single run",
+        description:
+          "Look at the performance of this run, compared to a trivial baseline",
+        detail: undefined,
+        query: undefined,
+      },
+      ...comparableQueryLabels,
+    ];
+    if (comparableQueryLabelsWithDefault.length < 1) {
+      throw new Error("No other queries available to compare with.");
+    }
+    const choice = await window.showQuickPick(comparableQueryLabelsWithDefault);
+
+    return choice?.query;
+  }
+
   /**
    * Updates the compare with source query. This ensures that all compare command invocations
    * when exactly 2 queries are selected always have the proper _from_ query. Always use