If I have "alter table foo alter COLUMN bar SET STATISTICS" in the line buffer,
it tab completes to add " TO", which is not legal.
The attached patch makes it not tab complete anything at all, which is at least not actively misleading.
I thought of having it complete "-1", "<integer>" so that it gives a clue about what is needed, but I didn't see any precedence for non-literal clue-giving and I did not want to try to create new precedence.
