planner: avoid double scan for index prefix col is (not) null

executor/set_test.go

@@ -830,6 +830,20 @@ func TestSetVar(t *testing.T) {
 	tk.MustExec("set global tidb_auto_analyze_partition_batch_size = 9999")
 	tk.MustQuery("select @@global.tidb_auto_analyze_partition_batch_size").Check(testkit.Rows("1024")) // max value is 1024
 
+	// test variable 'tidb_opt_prefix_index_single_scan'
+	// global scope
+	tk.MustQuery("select @@global.tidb_opt_prefix_index_single_scan").Check(testkit.Rows("1")) // default value
+	tk.MustExec("set global tidb_opt_prefix_index_single_scan = 0")
+	tk.MustQuery("select @@global.tidb_opt_prefix_index_single_scan").Check(testkit.Rows("0"))
+	tk.MustExec("set global tidb_opt_prefix_index_single_scan = 1")
+	tk.MustQuery("select @@global.tidb_opt_prefix_index_single_scan").Check(testkit.Rows("1"))
+	// session scope
+	tk.MustQuery("select @@session.tidb_opt_prefix_index_single_scan").Check(testkit.Rows("1")) // default value
+	tk.MustExec("set session tidb_opt_prefix_index_single_scan = 0")
+	tk.MustQuery("select @@session.tidb_opt_prefix_index_single_scan").Check(testkit.Rows("0"))
+	tk.MustExec("set session tidb_opt_prefix_index_single_scan = 1")
+	tk.MustQuery("select @@session.tidb_opt_prefix_index_single_scan").Check(testkit.Rows("1"))
+
 	// test tidb_opt_range_max_size
 	tk.MustQuery("select @@tidb_opt_range_max_size").Check(testkit.Rows("67108864"))
 	tk.MustExec("set global tidb_opt_range_max_size = -1")

planner/core/exhaust_physical_plans.go

@@ -1148,7 +1148,7 @@ func (p *LogicalJoin) constructInnerIndexScanTask(
 		cop.commonHandleCols = ds.commonHandleCols
 	}
 	is.initSchema(append(path.FullIdxCols, ds.commonHandleCols...), cop.tablePlan != nil)
-	indexConds, tblConds := ds.splitIndexFilterConditions(filterConds, path.FullIdxCols, path.FullIdxColLens, ds.tableInfo)
+	indexConds, tblConds := ds.splitIndexFilterConditions(filterConds, path.FullIdxCols, path.FullIdxColLens)
 	if maxOneRow {
 		// Theoretically, this line is unnecessary because row count estimation of join should guarantee rowCount is not larger
 		// than 1.0; however, there may be rowCount larger than 1.0 in reality, e.g, pseudo statistics cases, which does not reflect

planner/core/find_best_task.go

@@ -1118,7 +1118,7 @@ func (ds *DataSource) convertToIndexMergeScan(prop *property.PhysicalProperty, c
 
 func (ds *DataSource) convertToPartialIndexScan(prop *property.PhysicalProperty, path *util.AccessPath) (indexPlan PhysicalPlan) {
 	is := ds.getOriginalPhysicalIndexScan(prop, path, false, false)
-	// TODO: Consider using isCoveringIndex() to avoid another TableRead
+	// TODO: Consider using indexCoveringColumns() to avoid another TableRead
 	indexConds := path.IndexFilters
 	if indexConds != nil {
 		var selectivity float64
@@ -1281,33 +1281,80 @@ func extractFiltersForIndexMerge(sc *stmtctx.StatementContext, client kv.Client,
 	return
 }
 
-func indexCoveringCol(col *expression.Column, indexCols []*expression.Column, idxColLens []int) bool {
+func indexColsCoveringCol(col *expression.Column, indexCols []*expression.Column, idxColLens []int, ignoreLen bool) bool {
 	for i, indexCol := range indexCols {
-		isFullLen := idxColLens[i] == types.UnspecifiedLength || idxColLens[i] == col.RetType.GetFlen()
-		if indexCol != nil && col.EqualByExprAndID(nil, indexCol) && isFullLen {
+		if indexCol == nil || !col.EqualByExprAndID(nil, indexCol) {
+			continue
+		}
+		if ignoreLen || idxColLens[i] == types.UnspecifiedLength || idxColLens[i] == col.RetType.GetFlen() {
 			return true
 		}
 	}
 	return false
 }
 
-func (ds *DataSource) isCoveringIndex(columns, indexColumns []*expression.Column, idxColLens []int, tblInfo *model.TableInfo) bool {
+func (ds *DataSource) indexCoveringColumn(column *expression.Column, indexColumns []*expression.Column, idxColLens []int, ignoreLen bool) bool {
+	if ds.tableInfo.PKIsHandle && mysql.HasPriKeyFlag(column.RetType.GetFlag()) {
+		return true
+	}
+	if column.ID == model.ExtraHandleID {
+		return true
+	}
+	coveredByPlainIndex := indexColsCoveringCol(column, indexColumns, idxColLens, ignoreLen)
+	coveredByClusteredIndex := indexColsCoveringCol(column, ds.commonHandleCols, ds.commonHandleLens, ignoreLen)
+	if !coveredByPlainIndex && !coveredByClusteredIndex {
+		return false
+	}
+	isClusteredNewCollationIdx := collate.NewCollationEnabled() &&
+		column.GetType().EvalType() == types.ETString &&
+		!mysql.HasBinaryFlag(column.GetType().GetFlag())
+	if !coveredByPlainIndex && coveredByClusteredIndex && isClusteredNewCollationIdx && ds.table.Meta().CommonHandleVersion == 0 {
+		return false
+	}
+	return true
+}
+
+func (ds *DataSource) indexCoveringColumns(columns, indexColumns []*expression.Column, idxColLens []int) bool {
 	for _, col := range columns {
-		if tblInfo.PKIsHandle && mysql.HasPriKeyFlag(col.RetType.GetFlag()) {
-			continue
+		if !ds.indexCoveringColumn(col, indexColumns, idxColLens, false) {
+			return false
 		}
-		if col.ID == model.ExtraHandleID {
-			continue
+	}
+	return true
+}
+
+func (ds *DataSource) indexCoveringCondition(condition expression.Expression, indexColumns []*expression.Column, idxColLens []int) bool {
+	switch v := condition.(type) {
+	case *expression.Column:
+		return ds.indexCoveringColumn(v, indexColumns, idxColLens, false)
+	case *expression.ScalarFunction:
+		// Even if the index only contains prefix `col`, the index can cover `col is null`.
+		if v.FuncName.L == ast.IsNull {
+			if col, ok := v.GetArgs()[0].(*expression.Column); ok {
+				return ds.indexCoveringColumn(col, indexColumns, idxColLens, true)
+			}
 		}
-		coveredByPlainIndex := indexCoveringCol(col, indexColumns, idxColLens)
-		coveredByClusteredIndex := indexCoveringCol(col, ds.commonHandleCols, ds.commonHandleLens)
-		if !coveredByPlainIndex && !coveredByClusteredIndex {
-			return false
+		for _, arg := range v.GetArgs() {
+			if !ds.indexCoveringCondition(arg, indexColumns, idxColLens) {
+				return false
+			}
 		}
-		isClusteredNewCollationIdx := collate.NewCollationEnabled() &&
-			col.GetType().EvalType() == types.ETString &&
-			!mysql.HasBinaryFlag(col.GetType().GetFlag())
-		if !coveredByPlainIndex && coveredByClusteredIndex && isClusteredNewCollationIdx && ds.table.Meta().CommonHandleVersion == 0 {
+		return true
+	}
+	return true
+}
+
+func (ds *DataSource) isSingleScan(indexColumns []*expression.Column, idxColLens []int) bool {
+	if !ds.ctx.GetSessionVars().OptPrefixIndexSingleScan || ds.colsRequiringFullLen == nil {
+		// ds.colsRequiringFullLen is set at (*DataSource).PruneColumns. In some cases we don't reach (*DataSource).PruneColumns
+		// and ds.colsRequiringFullLen is nil, so we fall back to ds.indexCoveringColumns(ds.schema.Columns, indexColumns, idxColLens).
+		return ds.indexCoveringColumns(ds.schema.Columns, indexColumns, idxColLens)
+	}
+	if !ds.indexCoveringColumns(ds.colsRequiringFullLen, indexColumns, idxColLens) {
+		return false
+	}
+	for _, cond := range ds.allConds {
+		if !ds.indexCoveringCondition(cond, indexColumns, idxColLens) {
 			return false
 		}
 	}
@@ -1575,11 +1622,17 @@ func matchIndicesProp(idxCols []*expression.Column, colLens []int, propItems []p
 	return true
 }
 
-func (ds *DataSource) splitIndexFilterConditions(conditions []expression.Expression, indexColumns []*expression.Column, idxColLens []int,
-	table *model.TableInfo) (indexConds, tableConds []expression.Expression) {
+func (ds *DataSource) splitIndexFilterConditions(conditions []expression.Expression, indexColumns []*expression.Column,
+	idxColLens []int) (indexConds, tableConds []expression.Expression) {
 	var indexConditions, tableConditions []expression.Expression
 	for _, cond := range conditions {
-		if ds.isCoveringIndex(expression.ExtractColumns(cond), indexColumns, idxColLens, table) {
+		var covered bool
+		if ds.ctx.GetSessionVars().OptPrefixIndexSingleScan {
+			covered = ds.indexCoveringCondition(cond, indexColumns, idxColLens)
+		} else {
+			covered = ds.indexCoveringColumns(expression.ExtractColumns(cond), indexColumns, idxColLens)
+		}
+		if covered {
 			indexConditions = append(indexConditions, cond)
 		} else {
 			tableConditions = append(tableConditions, cond)

planner/core/integration_test.go

@@ -7528,6 +7528,10 @@ func TestCorColRangeWithRangeMaxSize(t *testing.T) {
 	tk.MustExec("insert into t3 values (2), (4)")
 	tk.MustExec("insert into mysql.opt_rule_blacklist value(\"decorrelate\")")
 	tk.MustExec("admin reload opt_rule_blacklist")
+	defer func() {
+		tk.MustExec("delete from mysql.opt_rule_blacklist where name = \"decorrelate\"")
+		tk.MustExec("admin reload opt_rule_blacklist")
+	}()
 
 	// Correlated column in index range.
 	tk.MustExec("set @@tidb_opt_range_max_size=1000")
@@ -7693,3 +7697,38 @@ func TestOuterJoinEliminationForIssue18216(t *testing.T) {
 	tk.MustExec("select group_concat(c order by (select group_concat(c order by a) from t2 where a=t1.a)) from t1; ")
 	tk.MustQuery("select group_concat(c order by (select group_concat(c order by c) from t2 where a=t1.a), c desc) from t1;").Check(testkit.Rows("2,1,4,3"))
 }
+
+func TestNullConditionForPrefixIndex(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec(`CREATE TABLE t1 (
+  id char(1) DEFAULT NULL,
+  c1 varchar(255) DEFAULT NULL,
+  c2 text DEFAULT NULL,
+  KEY idx1 (c1),
+  KEY idx2 (c1,c2(5))
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin`)
+	tk.MustExec("create table t2(a int, b varchar(10), index idx(b(5)))")
+	tk.MustExec("set tidb_opt_prefix_index_single_scan = 1")
+	tk.MustExec("insert into t1 values ('a', '0xfff', '111111'), ('b', '0xfff', '222222'), ('c', '0xfff', ''), ('d', '0xfff', null)")
+	tk.MustExec("insert into t2 values (1, 'aaaaaa'), (2, 'bbb'), (3, ''), (4, null)")
+
+	var input []string
+	var output []struct {
+		SQL    string
+		Plan   []string
+		Result []string
+	}
+	integrationSuiteData := core.GetIntegrationSuiteData()
+	integrationSuiteData.LoadTestCases(t, &input, &output)
+	for i, tt := range input {
+		testdata.OnRecord(func() {
+			output[i].SQL = tt
+			output[i].Plan = testdata.ConvertRowsToStrings(tk.MustQuery("explain format='brief' " + tt).Rows())
+			output[i].Result = testdata.ConvertRowsToStrings(tk.MustQuery(tt).Sort().Rows())
+		})
+		tk.MustQuery("explain format='brief' " + tt).Check(testkit.Rows(output[i].Plan...))
+		tk.MustQuery(tt).Sort().Check(testkit.Rows(output[i].Result...))
+	}
+}

planner/core/logical_plans.go

@@ -1220,6 +1220,10 @@ type DataSource struct {
 	// contain unique index and the first field is tidb_shard(),
 	// such as (tidb_shard(a), a ...), the fields are more than 2
 	containExprPrefixUk bool
+
+	// colsRequiringFullLen is the columns that must be fetched with full length.
+	// It is used to decide whether single scan is enough when reading from an index.
+	colsRequiringFullLen []*expression.Column
 }
 
 // ExtractCorrelatedCols implements LogicalPlan interface.
@@ -1343,7 +1347,7 @@ func (ds *DataSource) Convert2Gathers() (gathers []LogicalPlan) {
 			path.FullIdxCols, path.FullIdxColLens = expression.IndexInfo2Cols(ds.Columns, ds.schema.Columns, path.Index)
 			path.IdxCols, path.IdxColLens = expression.IndexInfo2PrefixCols(ds.Columns, ds.schema.Columns, path.Index)
 			// If index columns can cover all of the needed columns, we can use a IndexGather + IndexScan.
-			if ds.isCoveringIndex(ds.schema.Columns, path.FullIdxCols, path.FullIdxColLens, ds.tableInfo) {
+			if ds.isSingleScan(path.FullIdxCols, path.FullIdxColLens) {
 				gathers = append(gathers, ds.buildIndexGather(path))
 			}
 			// TODO: If index columns can not cover the schema, use IndexLookUpGather.
@@ -1548,7 +1552,7 @@ func (ds *DataSource) deriveIndexPathStats(path *util.AccessPath, _ []expression
 		}
 	}
 	var indexFilters []expression.Expression
-	indexFilters, path.TableFilters = ds.splitIndexFilterConditions(path.TableFilters, path.FullIdxCols, path.FullIdxColLens, ds.tableInfo)
+	indexFilters, path.TableFilters = ds.splitIndexFilterConditions(path.TableFilters, path.FullIdxCols, path.FullIdxColLens)
 	path.IndexFilters = append(path.IndexFilters, indexFilters...)
 	// If the `CountAfterAccess` is less than `stats.RowCount`, there must be some inconsistent stats info.
 	// We prefer the `stats.RowCount` because it could use more stats info to calculate the selectivity.

planner/core/rule_column_pruning.go

@@ -312,6 +312,14 @@ func (ds *DataSource) PruneColumns(parentUsedCols []*expression.Column, opt *log
 
 	originSchemaColumns := ds.schema.Columns
 	originColumns := ds.Columns
+
+	ds.colsRequiringFullLen = make([]*expression.Column, 0, len(used))
+	for i, col := range ds.schema.Columns {
+		if used[i] || (ds.containExprPrefixUk && expression.GcColumnExprIsTidbShard(col.VirtualExpr)) {
+			ds.colsRequiringFullLen = append(ds.colsRequiringFullLen, col)
+		}
+	}
+
 	for i := len(used) - 1; i >= 0; i-- {
 		if !used[i] && !exprUsed[i] {
 			// If ds has a shard index, and the column is generated column by `tidb_shard()`

planner/core/stats.go

@@ -277,7 +277,7 @@ func (ds *DataSource) derivePathStatsAndTryHeuristics() error {
 			path.IsSingleScan = true
 		} else {
 			ds.deriveIndexPathStats(path, ds.pushedDownConds, false)
-			path.IsSingleScan = ds.isCoveringIndex(ds.schema.Columns, path.FullIdxCols, path.FullIdxColLens, ds.tableInfo)
+			path.IsSingleScan = ds.isSingleScan(path.FullIdxCols, path.FullIdxColLens)
 		}
 		// Try some heuristic rules to select access path.
 		if len(path.Ranges) == 0 {

planner/core/testdata/integration_suite_in.json

@@ -1011,5 +1011,20 @@
       "set @@tidb_opt_range_max_size = 300",
       "explain format='brief' select /*+ inl_join(t1) */ * from t1 join t2 on t1.a = t2.e where t1.b > t2.f and t1.b < t2.f + 10"
     ]
+  },
+  {
+    "name": "TestNullConditionForPrefixIndex",
+    "cases": [
+      "select count(1) from t1 where c1 = '0xfff' and c2 is not null",
+      "select count(1) from t1 where c1 = '0xfff' and c2 is null",
+      "select count(1) from t1 where c1 >= '0xfff' and c2 is not null",
+      "select count(1) from t1 where c1 >= '0xfff' and c2 is null",
+      "select c2 from t1 use index(idx2) where c1 = '0xfff' and c2 is not null",
+      "select c2 from t1 use index(idx2) where c1 = '0xfff' and c2 is null",
+      "select c2 from t1 use index(idx2) where c1 >= '0xfff' and c2 is not null",
+      "select c2 from t1 use index(idx2) where c1 >= '0xfff' and c2 is null",
+      "select b from t2 where b is not null",
+      "select b from t2 where b is null"
+    ]
   }
 ]