Drizzle ORM Guidelines

Use $type() for Branded Strings, Not customType

When you need a column with a branded TypeScript type but no actual data transformation, use $type<T>() instead of customType .

The Rule

If toDriver and fromDriver would be identity functions (x) => x , use $type<T>() instead.

Why

Even with identity functions, customType still invokes mapFromDriverValue on every row:

// drizzle-orm/src/utils.ts - runs for EVERY column of EVERY row const rawValue = row[columnIndex]!; const value = rawValue === null ? null : decoder.mapFromDriverValue(rawValue);

Query 1000 rows with 3 date columns = 3000 function calls doing nothing.

Bad Pattern

// Runtime overhead for identity functions customType<{ data: DateTimeString; driverParam: DateTimeString }>({ dataType: () => 'text', toDriver: (value) => value, // called on every write fromDriver: (value) => value, // called on every read });

Good Pattern

// Zero runtime overhead - pure type assertion text().$type<DateTimeString>();

$type<T>() is a compile-time-only type override:

// drizzle-orm/src/column-builder.ts $type<TType>(): $Type<this, TType> { return this as $Type<this, TType>; }

When to Use customType

Only when data genuinely transforms between app and database:

// JSON: object ↔ string - actual transformation customType<{ data: UserPrefs; driverParam: string }>({ toDriver: (value) => JSON.stringify(value), fromDriver: (value) => JSON.parse(value), });

Keep Data in Intermediate Representation

Prefer keeping data serialized (strings) through the system, parsing only at the edges (UI components).

The principle: If data enters serialized and leaves serialized, keep it serialized in the middle. Parse at the edges where you actually need the rich representation.

Example: DateTimeString

Instead of parsing DateTimeString into Temporal.ZonedDateTime at the database layer:

// Bad: parse on every read, re-serialize at API boundaries customType<{ data: Temporal.ZonedDateTime; driverParam: string }>({ fromDriver: (value) => fromDateTimeString(value), });

Keep it as a string until the UI actually needs it:

// Good: string stays string, parse only in date-picker component text().$type<DateTimeString>();

// In UI component: const temporal = fromDateTimeString(row.createdAt); // After edit: const updated = toDateTimeString(temporal);