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The "optional" portion is this part of the spec, not the time part.

> implementations can guarantee additional monotonicity via the concepts covered in this section

The “time part” is actually two different parts: the required millisecond-level ordering and the optional use of part of rand_a (which postgres does) to provide higher-resolution (nanosecond, in the postgres case) time ordering when combined with the required portion.

So, no, the “time part” of the postgres implementation is, in part, one of the options discussed in the spec, not merely the “time part” required in the spec.

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