Row set index base class used when indexing rows within a row
set for a row set reader. Keeps track of the current position,
which starts before the first row, meaning that the client
must call next() to advance to the first row.
Ordinal index within the batch or array. Increments from -1.
(The position before the first item.)
Identifies the logical row number of top-level records,
or the array element for arrays. Actual physical
index may be different if an indirection layer is in use.