Add SETTINGS clause support for SYSTEM queries

Parse and explain SETTINGS for SYSTEM commands like
"SYSTEM FLUSH DISTRIBUTED ... SETTINGS ...".

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

ast/ast.go

@@ -959,6 +959,7 @@ type SystemQuery struct {
 	Table                string         `json:"table,omitempty"`
 	OnCluster            string         `json:"on_cluster,omitempty"`
 	DuplicateTableOutput bool           `json:"duplicate_table_output,omitempty"` // True for commands that need database/table output twice
+	Settings             []*SettingExpr `json:"settings,omitempty"`
 }
 
 func (s *SystemQuery) Pos() token.Position { return s.Position }

internal/explain/statements.go

@@ -837,6 +837,10 @@ func explainSystemQuery(sb *strings.Builder, n *ast.SystemQuery, indent string)
 			children *= 2
 		}
 	}
+	// Settings adds a child
+	if len(n.Settings) > 0 {
+		children++
+	}
 	if children > 0 {
 		fmt.Fprintf(sb, "%sSYSTEM query (children %d)\n", indent, children)
 		if n.Database != "" {
@@ -854,6 +858,10 @@ func explainSystemQuery(sb *strings.Builder, n *ast.SystemQuery, indent string)
 				fmt.Fprintf(sb, "%s Identifier %s\n", indent, n.Table)
 			}
 		}
+		// Output Set for settings
+		if len(n.Settings) > 0 {
+			fmt.Fprintf(sb, "%s Set\n", indent)
+		}
 	} else {
 		fmt.Fprintf(sb, "%sSYSTEM query\n", indent)
 	}

parser/parser.go

@@ -7026,6 +7026,12 @@ func (p *Parser) parseSystem() *ast.SystemQuery {
 		}
 	}
 
+	// Parse optional SETTINGS clause
+	if p.currentIs(token.SETTINGS) {
+		p.nextToken() // skip SETTINGS
+		sys.Settings = p.parseSettingsList()
+	}
+
 	return sys
 }
 

parser/testdata/03030_system_flush_distributed_settings/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt16": true
-  }
-}
+{}