Currently, we have Field and ColumnSchema to access column data in a
row. These two classes are mostly the same. So we should unify these to
one class. Now, Field has offset information, which is an row attribute,
so we remove offset in Field.
RowCursor now has some logic which belong to Schema, so in this patch I
add Schema attribute to RowCursor to make RowCursor simple. After this
change, only Schema will handle Field/ColumnSchema.
I extract some logic from RowCursor to be/src/olap/row.h, then we can
use same logic to handle different types of row. Each type of row has
same function that to get Cell of this row. A cell represent a column
content with a null indicator.
Use RowwiseIterator to uniform all data fetch in storage engine.
All objects in storage engine can be read in iterator format.
For example: Segment, Rowset.
This patch implement two generic iterators: UnionRowwiseIterator,
MergeRowwiseIterator. These two class will add iterator as its inputs.
To implement iterators, we define a new class RowBlockV2, all data read
from iterator is in this format. We define a new class other than use
old version's RowBlock is because we want to keep old code work
normally.
