add tests

cmd/explaintest/r/explain_indexmerge.result

@@ -97,6 +97,13 @@ label = "cop"
 }
 
 set session tidb_enable_index_merge = off;
+explain select /*+ use_index_merge(t, primary, tb, tc) */ * from t where a <= 500000 or b <= 1000000 or c <= 3000000;
+id	estRows	task	access object	operator info
+IndexMerge_9	3560000.00	root		
+├─TableRangeScan_5(Build)	500000.00	cop[tikv]	table:t	range:[-inf,500000], keep order:false
+├─IndexRangeScan_6(Build)	1000000.00	cop[tikv]	table:t, index:tb(b)	range:[-inf,1000000], keep order:false
+├─IndexRangeScan_7(Build)	3000000.00	cop[tikv]	table:t, index:tc(c)	range:[-inf,3000000], keep order:false
+└─TableRowIDScan_8(Probe)	3560000.00	cop[tikv]	table:t	keep order:false
 explain select /*+ use_index_merge(t, tb, tc) */ * from t where b < 50 or c < 5000000;
 id	estRows	task	access object	operator info
 IndexMerge_8	4999999.00	root		

cmd/explaintest/t/explain_indexmerge.test

@@ -20,6 +20,7 @@ explain select * from t where (b < 10000 or c < 10000) and (a < 10 or d < 10) an
 explain format="dot" select * from t where (a < 50 or b < 50) and f > 100;
 set session tidb_enable_index_merge = off;
 # be forced to use IndexMerge
+explain select /*+ use_index_merge(t, primary, tb, tc) */ * from t where a <= 500000 or b <= 1000000 or c <= 3000000;
 explain select /*+ use_index_merge(t, tb, tc) */ * from t where b < 50 or c < 5000000;
 explain select /*+ use_index_merge(t, tb, tc) */ * from t where (b < 10000 or c < 10000) and (a < 10 or d < 10) and f < 10;
 explain select /*+ use_index_merge(t, tb) */ * from t where b < 50 or c < 5000000;

planner/core/stats.go

@@ -425,9 +425,7 @@ func (ds *DataSource) generateIndexMergeOrPaths() {
 				sel = SelectionFactor
 			}
 			possiblePath.CountAfterAccess = sel * ds.tableStats.RowCount
-			if possiblePath != nil {
-				ds.possibleAccessPaths = append(ds.possibleAccessPaths, possiblePath)
-			}
+			ds.possibleAccessPaths = append(ds.possibleAccessPaths, possiblePath)
 		}
 	}
 }