planner: quick fix the panic issue when restoring hints from a plan (#48640)

ref #46791

Author: qw4990

pkg/planner/core/hints.go

@@ -108,7 +108,10 @@ func getJoinHints(sctx sessionctx.Context, joinType string, parentOffset int, no
 		}
 		var dbName, tableName *model.CIStr
 		if blockOffset != parentOffset {
-			blockAsNames := *(sctx.GetSessionVars().PlannerSelectBlockAsName.Load())
+			var blockAsNames []ast.HintTable
+			if p := sctx.GetSessionVars().PlannerSelectBlockAsName.Load(); p != nil {
+				blockAsNames = *p
+			}
 			if blockOffset >= len(blockAsNames) {
 				continue
 			}

pkg/planner/core/logical_plan_builder.go

@@ -549,7 +549,11 @@ func (b *PlanBuilder) buildResultSetNode(ctx context.Context, node ast.ResultSet
 			}
 		}
 		// `TableName` is not a select block, so we do not need to handle it.
-		if plannerSelectBlockAsName := *(b.ctx.GetSessionVars().PlannerSelectBlockAsName.Load()); len(plannerSelectBlockAsName) > 0 && !isTableName {
+		var plannerSelectBlockAsName []ast.HintTable
+		if p := b.ctx.GetSessionVars().PlannerSelectBlockAsName.Load(); p != nil {
+			plannerSelectBlockAsName = *p
+		}
+		if len(plannerSelectBlockAsName) > 0 && !isTableName {
 			plannerSelectBlockAsName[p.SelectBlockOffset()] = ast.HintTable{DBName: p.OutputNames()[0].DBName, TableName: p.OutputNames()[0].TblName}
 		}
 		// Duplicate column name in one table is not allowed.
@@ -721,7 +725,10 @@ func extractTableAlias(p Plan, parentOffset int) *hintTableInfo {
 			}
 		}
 		blockOffset := p.SelectBlockOffset()
-		blockAsNames := *(p.SCtx().GetSessionVars().PlannerSelectBlockAsName.Load())
+		var blockAsNames []ast.HintTable
+		if p := p.SCtx().GetSessionVars().PlannerSelectBlockAsName.Load(); p != nil {
+			blockAsNames = *p
+		}
 		// For sub-queries like `(select * from t) t1`, t1 should belong to its surrounding select block.
 		if blockOffset != parentOffset && blockAsNames != nil && blockAsNames[blockOffset].TableName.L != "" {
 			blockOffset = parentOffset

pkg/planner/core/rule_join_reorder.go

@@ -399,7 +399,10 @@ func (s *baseSingleGroupJoinOrderSolver) generateLeadingJoinGroup(curJoinGroup [
 	var leadingJoinGroup []LogicalPlan
 	leftJoinGroup := make([]LogicalPlan, len(curJoinGroup))
 	copy(leftJoinGroup, curJoinGroup)
-	queryBlockNames := *(s.ctx.GetSessionVars().PlannerSelectBlockAsName.Load())
+	var queryBlockNames []ast.HintTable
+	if p := s.ctx.GetSessionVars().PlannerSelectBlockAsName.Load(); p != nil {
+		queryBlockNames = *p
+	}
 	for _, hintTbl := range hintInfo.leadingJoinOrder {
 		match := false
 		for i, joinGroup := range leftJoinGroup {