DECIMAL Types in tsql

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I have a Table with about 200Mio Rows and multiple Columns of Datatype DECIMAL(p,s) with varying precision/scales. Now, as far as i understand, DECIMAL(p,s) is a fixed size column, with a size depending on the precision, see:

https://docs.microsoft.com/en-us/sql/t-sql/data-types/decimal-and-numeric-transact-sql?view=sql-server-ver16

Now, when altering the table and changing a column from DECIMAL(15,2) to DECIMAL(19,6), for example, i would have expected there to be almost no work to be done on the side of the SQL-Sever as the required bytes to store the value are the same, yet the altering itself does take a long time - so what exactly does the server do when i execute the alter statement?

Also, is there any benefit (other than having constraints on a column) to storing a DECIMAL(15,2) instead of, for example, a DECIMAL(19,2)? It seems to me the storage requirements would be the same, but i could store larger values in the latter.

Thanks in advance!

decimal storage tsql byte
1 Answers

The precision and scale of a decimal / numeric type matters considerably.

As far as SQL Server is concerned, decimal(15,2) is a different data type to decimal(19,6), and is stored differently. You therefore cannot make the assumption that just because the overall storage requirements do not change, nothing else does.

SQL Server stores decimal data types in byte-reversed (little endian) format with the scale being the first incrementing value therefore changing the definition can require the data to be re-written, SQL Server will use an internal worktable to safely convert the data and update the values on every page.

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