In some cases, you can create more than one column index at the same time:
Each CREATE INDEX statement can create only one index.
Each connection can create only one index at a time.
If two connections issue CREATE INDEX statements on the same table, the first statement works; the other gets an error saying that only 1 writer is allowed.
If two connections issue CREATE INDEX statements on different tables, both proceed in parallel.
If two connections issue CREATE INDEX statements on different tables but both tables participate in the same join index, then only one CREATE INDEX works; the other gets an error saying that only 1 writer is allowed.
