Hi William, The empty column is needed to ensure that a given column is available for all rows.
As you may know, data is stored in HBase as KeyValues, meaning that the full row key is stored for each column value. This also implies that the row key is not stored at all unless there is at least one column stored. Now consider JDBC row which has an integer primary key, and several columns which are all null. In order to be able to store the primary key, a KeyValue needs to be stored to show that the row is present at all. This column is represented by the empty column that you've noticed. This allows doing a "SELECT * FROM TABLE" and receiving records for all rows, even those whose non-pk columns are null. The same issue comes up even if only one column is null for some (or all) records. A scan over Phoenix will include the empty column to ensure that rows that only consist of the primary key (and have null for all non-key columns) will be included in a scan result. - Gabriel On Mon, Dec 21, 2015 at 2:58 PM, 杨晗 <yhxx...@163.com> wrote: > hi all: > I'm reading phoenix src code recently, and i found > PRowImpl.toRowMutations() always adds an empty column named '_0' for > non-delete upsert. Why? > I read the comment but i didn't quite understand it. Might someone give > me an example that illustates why an empty column is ALWAYS necessary? > > > Further more, I have to access a phoenix table by both phoenix client and > hbase API in some cases. If i do not add this empty column explicitly by > hbase API, it is ok if i read this row by phoenix? > > > Thanks > -William
