Fix bug when DELETE ACID block is a DictionaryBlock

[djsagain]

This PR consists of two commits:

Before the first commit, Block.getChildren() was used to take apart the
DELETE ACID rowId block. That is always the wrong thing to do.
Fixed by using ColumnarRow to access the elements of the ACID rowId block.

This first commit also adds a comment warning developers not to use
Block.getChildren() so others don't make the same mistake of
calling the method.

The second commit replaces use of Block.getChildren in
HiveUpdateProcessor with ColumnarRow methods.

[findepi]

Should this check survive in the form of columnarRow.getFieldCount()==3 inside deleteRowsInternal?

[djsagain]

Good idea, added.

[findepi]

redundant

Suggested change

// Throw an exception if there are any null rows

[djsagain]

Removed.

[findepi]

deleteRows is not a variable or anything else known

Suggested change

	checkArgument(!columnarRow.isNull(position), "In the deleteRows block, found null position %s", position);
	checkArgument(!columnarRow.isNull(position), "In the delete rowIds, found null row at position %s", position);

[djsagain]

Replaced.

[findepi]

i think the important aspect is "delete after delete".

assuming it's correct, let's reflect this in a test name: testDeleteAfterDelete

[djsagain]

Good idea; renamed.

[findepi]

nit: lowercase count, true

[djsagain]

Changed.

[findepi]

nit: lowercase true

[findepi]

verification of this stage is a bit redundant (we already have some tests for delete), especially if we run this on Hive too. remove, or keep only on Trino.

[djsagain]

Changed to test only on Trino.

[findepi]

Was the bug triggered for DELETE without any predicate, or for a delete with a predicate too?

we should test both cases

[djsagain]

Good idea. I added a test that differs in this way:

            // A predicate sufficient to fool statistics-based optimization
            onTrino().executeQuery(format("DELETE FROM %s WHERE id != 2" + tableName));

[findepi]

I agree with @martint on https://github.com/trinodb/trino/pull/9295/files#r714984993

[djsagain]

All logging removed from this method.

[findepi]

cc @losipiuk

[djsagain]

Thanks for the prompt, close review, @findepi. I force pushed an update making all your suggested changes.

BTW, separating the WHERE clause out as a separate argument to the TestHiveTransactionalTable.verifySelect* methods, which I introduced, was a dumb idea. I will submit a no-semantic-change PR to remove them.

[findepi]

CI #8432

[findepi]

Merged as 87785b4, thanks!