In most cases skip indices should only be considered once other alternatives have been exhausted - specifically this advanced functionality should only be used after investigating other alternatives such as modifying the primary key, using projections, or materialized views. In general, only consider skip-indices if there is a strong correlation between the primary key and the targeted, non-primary column/expression. In the absence of any real correlation, the skipping index will match for most blocks - resulting in all granules being read into memory and evaluated. In this case, the index cost has been incurred for no benefit, effectively slowing the full table scan.
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All that said, if your love language is spreadsheets, the full specs are helpfully laid out below:
across the full spectrum of SI quantities, providing strong type safety, clearer code, and prevention of unit-related errors.