Migrating tables to In-Memory OLTP

One of the first things you might do when considering migrating a table to In-Memory OLTP, is to run the “Transaction Performance Analysis Overview” report:

BlogTPAO

Then you arrive at the following window:

BlogTPAnalysis

If you click on “Tables Analysis”, a sort of “magic quadrant” appears:

BlogQuadrant

The chart attempts to display both the best candidates and the degree of difficulty for migration. But there are a few problems with the “difficulty” rating of this internal query, and as a result, if we really want to know how easy/difficult the process might be, we’ll have to roll our own query.

The original query counts the following conditions (the list is out of order on purpose):

nullable columns
LOB data types, i.e. NVARCHAR(MAX)
unique indexes/constraints
default constraints
check constraints
UDT
foreign keys
triggers
index with type > 2
computed columns
sparse columns
IDENTITY <> 1, 1
assembly
unsupported data types, i.e. DATETIMEOFFSET
filestream columns

Some of those conditions are 100% supported in SQL 2016, without any issue, while others have variable levels of migration difficulty, and still others are not supported at all. But even if we remove the items that are completely supported, all of the remaining items have the same weight. That could be pretty misleading, and might cause you to rule out migrating a table to In-Memory that could potentially be an excellent candidate.

Now let’s look at the list in another way:

**************************
supported without any issues
**************************
nullable columns
LOB data types, i.e NVARCHAR(MAX)
unique indexes/constraints

**********************************************************************
supported with a range of migration difficulty (from no difficulty to very difficult)
**********************************************************************
default constraints
check constraints
UDT
foreign keys
triggers

index with type > 2
0 = Heap
1 = Clustered
2 = Nonclustered
3 = XML
4 = Spatial
5 = Clustered columnstore index
6 = Nonclustered columnstore index
7 = Nonclustered hash index

********************
unsupported/blocking
********************
computed columns
sparse columns
IDENTITY <> 1, 1
assembly
unsupported data types, i.e. DATETIMEOFFSET
filestream columns

My version of the script removes the checks for nullable and LOB columns, and also for UNIQUE indexes/constraints.

And for the remaining conditions, since they’re all weighted the same by virtue of counting them, I wanted to place them in different buckets. After running my script on a sample database, I can see that the AuditTrail table has the following potential migration “issues”:

BlogIssues

There are a total of 8 issues, although migrating default constraints, user-defined data types, and LOB columns will be easy. It’s the foreign keys that might prove difficult, potentially leading to a long chain of tables that would have to be migrated (because foreign key constraints on a memory-optimized table can only reference other memory-optimized tables).

We definitely have a much clearer picture of the potential migration difficulties. With this version of the script, you can make informed choices about which tables you might want to migrate to In-Memory OLTP.

Also note that computed columns are supported in SQL 2017, so this script could have some intelligence added to allow for that.

One thought on “Migrating tables to In-Memory OLTP

  1. Pingback: Finding Candidates For Memory-Optimized Tables – Curated SQL

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