Implementation:Apache Druid ExpressionMeta

Knowledge Sources	Apache_Druid
Domains	Web_Console, Explore_View
Last Updated	2026-02-10 10:00 GMT

Overview

An immutable value class representing a named SQL expression with optional alias, used as the metadata model for columns and dimensions in the explore view.

Description

The ExpressionMeta class wraps a SqlExpression with an optional alias (`as`) and derives a display name. It provides static methods for inflation from serialized values, creation from Column objects, and SQL type evaluation by matching column references against a column list. Instance methods support immutable changes via `change`, `changeAs`, `changeExpression`, and `applyRename` (which renames column references within the expression). The class implements `valueOf` for serialization and `equals` for identity comparison.

Usage

Used in the explore view as the model for split columns, show columns, and other expression-based parameters in module configurations. Instances are stored in parameter values and persisted to localStorage.

Code Reference

Source Location

• Repository: Apache Druid
• File: web-console/src/views/explore-view/models/expression-meta.ts
• Lines: 1-118

Signature

export interface ExpressionMetaValue {
  expression: SqlExpression;
  as?: string;
}

export class ExpressionMeta {
  static MAX_NAME_LENGTH: number;
  static inflate(value: any): ExpressionMeta | undefined;
  static inflateArray(value: any): ExpressionMeta[];
  static defaultNameFromExpression(expression: SqlExpression): string;
  static fromColumn(column: Column): ExpressionMeta;
  static evaluateSqlType(expression: SqlExpression, columns?: readonly Column[]): string | undefined;

  public readonly expression: SqlExpression;
  public readonly as?: string;
  public readonly name: string;

  constructor(value: ExpressionMetaValue);
  valueOf(): ExpressionMetaValue;
  equals(other: ExpressionMeta | undefined): boolean;
  change(newValues: Partial<ExpressionMetaValue>): this;
  changeAs(as: string): this;
  changeExpression(expression: SqlExpression): this;
  applyRename(rename: Map<string, string>): this;
  evaluateSqlType(columns?: readonly Column[]): string | undefined;
}

Import

import { ExpressionMeta } from '../models/expression-meta';

I/O Contract

Inputs

Name	Type	Required	Description
value	ExpressionMetaValue	Yes	An object containing a SqlExpression and optional alias string
column	Column	Yes	A druid-query-toolkit Column to create an ExpressionMeta from (static factory)
rename	Map<string, string>	Yes	A column name rename mapping to apply to the expression
columns	readonly Column[]	No	Column metadata for SQL type evaluation

Outputs

Name	Type	Description
(return)	ExpressionMeta	A new or inflated ExpressionMeta instance
(return)	ExpressionMeta[]	An array of inflated instances from serialized data (inflateArray)
(return)	undefined	The SQL type of the expression if it refers to a known column

Usage Examples

Create from a column

import { ExpressionMeta } from '../models/expression-meta';

const meta = ExpressionMeta.fromColumn({ name: 'channel', sqlType: 'VARCHAR', nativeType: 'STRING' });
// meta.name === 'channel'
// meta.expression === C('channel')

Rename columns in an expression

const renamed = meta.applyRename(new Map([['channel', 'source']]));
// renamed.expression now references 'source' instead of 'channel'

Inflate from serialized data

const inflated = ExpressionMeta.inflate({ expression: '"channel"', as: 'Source' });
// inflated.name === 'Source'

Related Pages

• Environment:Apache_Druid_Web_Console_Development