Updating partition key column realty show whos still dating
Prior to SQL Server 2012 when you add a new non-NULLable column with default values to an existing table a size-of data operation occurs: every row in the table is updated to add the default value of the new column.
For small tables this is insignificant, but for large tables this can be so problematic as to completely prohibit the operation.
There is no measurable performance penalty from having a default value. The ‘default’ value is pushed into the row, even if the column was not modified.
Consider this update: Key Hash Value = (8194443284a0) Slot 1 Offset 0x60 Length 15 Record Type = PRIMARY_RECORD Record Attributes = NULL_BITMAP Record Size = 15 Memory Dump @0x000000000AB8A060 0000000000000000: 10000c00 02000000 83010000 020000†††††††††††††..............
The expression can include columns from the same table, constants, SQL functions, and user-defined PL/SQL functions. Some column types, such as clause for these segments that will override storage parameters specified at the table level.
After you create a table, you insert rows of data using SQL statements or using an Oracle bulk load utility.
Working with the application developer, consider the following guidelines when designing tables: Before creating a table, you should also determine whether to use integrity constraints.
Integrity constraints can be defined on the columns of a table to enforce the business rules of your database automatically.
This new behavior occurs automatically, no special syntax or setting is required, the engine will simply do the right thing.
But starting with SQL Server 2012 the operation is, in most cases, instantaneous: only the table metadata is changed, no rows are being updated.
Lets look at a simple example, we’ll create a table with some rows and then add a non-NULL column with default values.
So the update has trully materialized the default value in the row image.
I intentionally copied the output of DBCC PAGE for the next slot in the page, to show that the record with id=2 was unaffected, it continues to have a smaller size of 15 bytes and Column 3 has no physical length.