Functions & Aggregates

The most common way to extend DortDB is to add functions, operators, aggregates, and castables, grouped into an extension that you register on the engine.

Functions

A function has a unique name and an implementation. Mark it pure when it has no side effects and returns the same output for the same input (which lets the optimizer reason about it):

const myExtension = {
  functions: [{ name: 'double', impl: (v: number) => v * 2, pure: true }],
};

const db = new DortDB({
  mainLang: SQL(),
  optimizer: { rules: defaultRules },
  extensions: [myExtension],
});

db.query('SELECT double(x) AS d FROM t');
// with t = [{ x: 5 }, { x: 7 }] → [{ d: 10 }, { d: 14 }]

Aggregates

An aggregate is expressed as three parts: init to create the starting state, step to fold in each value, and result to extract the final value:

const myExtension = {
  aggregates: [
    {
      name: 'product',
      init: () => 1,
      step: (state: number, v: number) => state * v,
      result: (state: number) => state,
    },
  ],
};

db.query('SELECT product(x) AS p FROM t');
// with t = [{ x: 5 }, { x: 7 }] → [{ p: 35 }]

By default aggregates ignore nulls; set includeNulls: true to fold them in. An optional stepInverse can speed up window-style use.

Operators and castables

• Operators ({ name, impl }) back query operators such as + or like. Only languages that support user-defined operators (e.g. SQL) will pick them up.
• Castables ({ name, convert }) back cast keywords, as in SQL's CAST(val AS mytype).

Scoping and namespacing

Set scope to restrict an extension to specific languages, and schema to namespace all of its members under a prefix:

const xqueryOnly = {
  scope: ['xquery'],
  aggregates: [
    /* ... only visible to XQuery ... */
  ],
};

Worked example: the datetime extension

The provided @dortdb/datetime extension bundles date/time helpers and is a good reference for how an extension is packaged:

import { datetime } from '@dortdb/datetime';

const db = new DortDB({
  mainLang: SQL(),
  optimizer: { rules: defaultRules },
  extensions: [datetime],
});

db.query(
  "SELECT date.extract(date.sub(now(), interval('3 years')), 'year') AS y FROM t",
);