The order of the columns in a table affects performance.
Columns in a row are accessed sequentially in the order of their creation. For example, to access columns at the end of a row, the database server traverses the columns that appear earlier in the row. Order your columns so that narrow and/or frequently accessed columns are placed before seldom accessed and/or wider columns in the table.
Wide columns are columns greater than 15 bytes in size, or LONG data types (for example, LONG VARCHAR), or columns defined as XML. Primary key columns are always stored at the beginning of row.