Fix: no table selected

[bajinsheng]

When no table is selected, the execution should be aborted to avoid execution errors in QPG.

[mrigger]

Rather than modifying the existing method, perhaps we could use a getRandomTableOrBailout method such as the one below that throws an exception when no table exists? In many contexts, it might be clear that a table exists, and not throwing an exception in such cases might help detect otherwise overlooked bugs (in SQLancer).

[bajinsheng]

I think it should be a bug as all dbs assume the returned list has at least one table and directly operate the object.
https://github.com/sqlancer/sqlancer/search?q=getRandomTable

[bajinsheng]

I cannot remember it exactly, but it should be some corner cases in which no table is returned, but there is no bug.

[bajinsheng]

Like this, it gets a random table and assumes the returned list is not empty:

sqlancer/src/sqlancer/cockroachdb/gen/CockroachDBShowGenerator.java

Line 25 in 695dda5

sb.append(globalState.getSchema().getRandomTable(t -> !t.isView()).getName());

But its assumption is not always true. It is a potential bug that was not exposed before.

[bajinsheng]

We can close it as it is, and apply this fix when observing this issue again.

[mrigger]

I think it should be a bug as all dbs assume the returned list has at least one table and directly operate the object.

Not sure if I would call this a bug - all implementations relying on this method indeed assume that at least one table exists, and I can't think of a circumstance where this doesn't hold. Did QPG result in a corner case where this assumption no longer holds? If so, perhaps we can rename getRandomTable to getRandomTableOrBailout to follow the naming convention in this class?

[mrigger]

We can close it as it is, and apply this fix when observing this issue again.

Okay, sounds good.