Question

I’ll use this simple query as an example:
Select MyColumn From MyTable Where PrimaryKeyColumn = 10.

Let’s assume the table has no clustered index and there is a primary key on PrimaryKeyColumn.

If I understand correctly, the query plan will include an index seek on the primary key and a bookmark lookup on the table, using some sort of row pointer. I have two questions:

1) What is this row pointer?

2) How efficient is finding a row in the table using this row pointer?

Thanks very much.

Answer 1

The bookmark on a heap (a table w/o a clustered index) is a physical address value (fileid:pageid:slotid). Looking up the bookmark is really fast, faster than a key seek in fact, but one may land on a ‘forwarded record‘, which left in place another bookmark to follow, when you rinse cycle and repeat the process. Having plans that do non-clustered index scans that would require large volume lookups may trigger the ‘index tipping point‘.