Sql-server – Best practices for selecting partition boundary ranges to maintain empty partitions at both end

partitioningsql serversql-server-2008-r2

I have partitioned the table based on a date datatype partitioning column.

The data for the column starts at "2014-01-01" and ends at "2015-01-01". I know that, the partitions should have empty partitions at left and right ends. So, I selected the following boundary ranges while creating the partition function. The partition function is shown below,

create partition function PfDateRange(date)
as range right for values('2014-04-01','2014-07-01','2014-10-01','2015-01-01')

I would like to hear some tips or good practices for selecting boundary ranges.

Also, at future the data may get inserted which were greater than the last boundary range. So how to maintain empty partition at the right end?

Thanks in advance.

Best Answer

Let me start by mentioning that these 4 boundaries result in following 5 partitions. Partitions 1 and 5 are unbounded one one side and partitions 2, 3, and 4 are constrained to quarterly data:

  • partition 1: values less than '2014-04-01'
  • partition 2: values >= '2014-04-01' and < '2014-07-01'
  • partition 3: values >= '2014-07-01' and < '2014-10-01'
  • partition 4: values >= '2014-10-01' and < '2015-01-01'
  • partition 5: values >= '2015-01-01'

The rationale for keeping (at least) the first and last partitions empty is to ensure expensive data movement is not required during partition maintenance of an incremental or sliding window scenario. If data must be moved to a different partition to accommodate new boundaries during MERGE or SPLIT, the logging is about 4 times greater than normal DDL operations. This is especially nasty with large tables, as is common with table partitioning.

Data movement is never required when an existing empty partition is split or when 2 adjacent empty partitions are merged. This is why the best practice is to plan for such, making the maintenance a no-brainer that can easily be automated.

That said, one can split a non-empty partition without overhead in some cases. For example, you could split the function to add the new '2015-04-01' boundary without data movement as long no existing rows are >= '2015-04-01'. But to also avoid an expensive scan during the offline operation, you need an index with the partitioning column as the high-order key column so that SQL Server can efficiently validate no data exists beyond the new boundary. So in your case, you could probably split the last partition in advance of the start of the next quarter without incurring overhead if the partitioning column is indexed (a common design pattern).

Assuming your objective is 4 quarterly partitions of historical data plus the current active quarter in a sliding window, I suggest bounded partitions for each of these 5 quarters plus another bounded one for the future quarter. This will result in 8 partitions when you include the unbounded first and last ones. 

CREATE PARTITION FUNCTION PfDateRange(date)
AS RANGE RIGHT FOR VALUES(
      '2014-01-01'
    , '2014-04-01'
    , '2014-07-01'
    , '2014-10-01'
    , '2015-01-01'
    , '2015-04-01'
    , '2015-07-01');

The reason one might want additional future empty partitions is to provide time to react in case partition maintenance is missed for some reason. Multiple future partitions are more important with more frequent maintenance (e.g. daily), and less so for quarterly maintenance.

After the start of the '2015-04-01' quarter, the partition maintenance script can:

  • switch '2014-01-01' to a and empty staging table and truncate staging
  • merge the '2014-01-01' partition to remove it
  • spilt at '2015-10-01' for the next future quarter

You may want to do a sanity check before the maintenance in case you have bad data outside of the expected range (less than '2014-01-01' or greater than '2015-07-01'). Alternatively, maintain a check constraint for the expected range so that only valid data can be inserted.

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