Row Structure

The way Oracle stores a row of a table is known as row structure. A row consists of two parts; one is header and the second is data. In the header part all the control information of that particular row is stored. Control information of a row consists of three parts. First is the links of row migration and row chaining. Secondly is the lock information. Locks are used so that no two transactions should update the row simultaneously. And the last thing is how many columns are there in that row. Every row in a table has same number of columns; so why there is a need for storing this information separately in each row. This is one of the features that only oracle provides. We will see the advantage of this in just a moment.

Next is the data portion. Data portion contains the actual data stored. Each entry of data has two parts. One is known as column length (CL) and the other is column value (CV). Oracle does not store the null values. This is the feature I talked about earlier. Suppose for a particular row there are five columns in which the last three has null values. Oracle does not store the null values altogether. Then how it knows that there is a null value? As I mentioned that oracle stores the number of columns for each row. So the value stored in the header will be 5 for that row but the actual values that are present are two. Oracle will realize here that the last three values are null. Real world databases do have a lot of null values. So oracle wastes a little space for storing the number of columns information in the header of every row but on the other hand it saves a lot of space by not storing the null values.