Unique columns

A column marked unique gets distinct values. Two modes, with a real trade-off between them.

Tracked (the default)

Synth resamples until it finds a fresh value, tracking everything generated so far. Values stay natural, but memory grows with the row count, and a column with fewer possible values than rows is an error rather than a silent duplicate:

synth: field "status" ran out of unique values after 3 rows; its value space is
too small for the row count (use unique=counter, or widen the field)

unique=counter

Derives distinctness from the record index instead. It costs a visible suffix (ivanov.dilnoza41293@gmail.com) and buys constant memory, any row count, and parallel generation — MakeParallel accepts it, where tracked unique fields must go through Make.

type Row struct {
    ID    uuid.UUID `synth:"pk"`                    // unique by construction
    Email string    `synth:"email,unique"`          // tracked, natural values
    Slug  string    `synth:"username,unique=counter"` // constant memory at 1B rows
}

fields:
  slug: { kind: username, unique: true, unique_mode: counter }

Choosing

	tracked	counter
Values	natural	natural plus a numeric suffix
Memory	grows with rows	constant
Row ceiling	the column's value space	none
MakeParallel	no — use Make	yes
Too-small value space	error	not possible