Fix a problem with "EXPLAIN QUERY PLAN SELECT count(*) FROM tbl".

FossilOrigin-Name: 9f9f32882501ac9b6e60f81195a64bdbf6e4497b

src/select.c

@@ -3618,6 +3618,32 @@ static void updateAccumulator(Parse *pParse, AggInfo *pAggInfo){
   sqlite3ExprCacheClear(pParse);
 }
 
+/*
+** Add a single OP_Explain instruction to the VDBE to explain a simple
+** count(*) query ("SELECT count(*) FROM pTab").
+*/
+#ifndef SQLITE_OMIT_EXPLAIN
+static void explainSimpleCount(
+  Parse *pParse,                  /* Parse context */
+  Table *pTab,                    /* Table being queried */
+  Index *pIdx                     /* Index used to optimize scan, or NULL */
+){
+  if( pParse->explain==2 ){
+    char *zEqp = sqlite3MPrintf(pParse->db, "SCAN TABLE %s %s%s(~%d rows)",
+        pTab->zName, 
+        pIdx ? "USING COVERING INDEX " : "",
+        pIdx ? pIdx->zName : "",
+        pTab->nRowEst
+    );
+    sqlite3VdbeAddOp4(
+        pParse->pVdbe, OP_Explain, pParse->iSelectId, 0, 0, zEqp, P4_DYNAMIC
+    );
+  }
+}
+#else
+# define explainSimpleCount(a,b,c)
+#endif
+
 /*
 ** Generate code for the SELECT statement given in the p argument.  
 **
@@ -4229,6 +4255,7 @@ int sqlite3Select(
         }
         sqlite3VdbeAddOp2(v, OP_Count, iCsr, sAggInfo.aFunc[0].iMem);
         sqlite3VdbeAddOp1(v, OP_Close, iCsr);
+        explainSimpleCount(pParse, pTab, pBest);
       }else
 #endif /* SQLITE_OMIT_BTREECOUNT */
       {