Memory Optimized Indexes | Hash vs Range Indexes
In SQL Server 2014 for In-Memory tables there are lot of changes in DDLs compared with normal Disk Based Tables. In-Memory Tables related changes we’ve seen in previous posts, check [here]. Here we will see Memory Optimized Index related changes and few important things to take care before designing your Tables and Indexes.
–> Some of the main points to note are:
1. Indexes on In-Memory tables must be created inline with CREATE TABLE DDL script only.
2. These Indexes are not persisted on Disk and reside only in memory, thus they are not logged. As these Indexes are not persistent so they are re-created whenever SQL Server is restarted. Thus In-Memory tables DO NOT support Clustered Indexes.
3. Only two types of Indexes can be created on In-Memory tables, i.e. Non Clustered Hash Index and Non Clustered Index (aka Range Index). So there is no bookmark lookup.
4. These Non Clustered Indexes are inherently Covering, and all columns are automatically INCLUDED in the Index.
5. Total there can be MAX 8 Non Clustered Indexes created on an In-Memory table.
–> Here we will see how Query Optimizer uses Hash & Range Indexes to process query and return results:
1. Hash Indexes: are used for Point Lookups or Seeks. Are optimized for index seeks on equality predicates and also support full index scans. Thus these will only perform better when the predicate clause contains only equality predicate (=).
2. Range Indexes: are used for Range Scans and Ordered Scans. Are optimized for index scans on inequality predicates, such as greater than or less than, as well as sort order. Thus these will only preform better when the predicate clause contains only inequality predicates (>, <, =, BETWEEN).
–> Let’s check this by some hands-on code. We will create 2 similar In-Memory tables, one with Range Index and another with Hash Index:
-- Create In-Memory Table with simple NonClustered Index (a.k.a Range Index): CREATE TABLE dbo.MemOptTable_With_NC_Range_Index ( ID INT NOT NULL PRIMARY KEY NONCLUSTERED, VarString VARCHAR(200) NOT NULL, DateAdded DATETIME NOT NULL ) WITH ( MEMORY_OPTIMIZED = ON, DURABILITY = SCHEMA_AND_DATA ) GO -- Create In-Memory Table with NonClustered Hash Index: CREATE TABLE dbo.MemOptTable_With_NC_Hash_Index ( ID INT NOT NULL PRIMARY KEY NONCLUSTERED HASH WITH (BUCKET_COUNT = 10000), VarString VARCHAR(200) NOT NULL, DateAdded DATETIME NOT NULL ) WITH ( MEMORY_OPTIMIZED = ON, DURABILITY = SCHEMA_AND_DATA ) GO
–> Now we will Insert about 10k records on both the tables, so that we have good numbers of records to compare:
DECLARE @i INT = 1 WHILE @i <= 10000 BEGIN INSERT INTO dbo.MemOptTable_With_NC_Range_Index VALUES(@i, REPLICATE('a', 200), GETDATE()) INSERT INTO dbo.MemOptTable_With_NC_Hash_Index VALUES(@i, REPLICATE('a', 200), GETDATE()) SET @i = @i+1 END
–> Now check the Execution Plan by using equality Operator (=) on both the tables:
SELECT * FROM MemOptTable_With_NC_Hash_Index WHERE ID = 5000 -- 4% SELECT * FROM MemOptTable_With_NC_Range_Index WHERE ID = 5000 -- 96%
You will see in the Execution Plan image below that Equality Operator with Hash Index Costs you only 4%, but Range Index Costs you 96%.
–> Now check the Execution Plan by using inequality Operator (BETWEEN) on both the tables:
SELECT * FROM MemOptTable_With_NC_Hash_Index WHERE ID BETWEEN 5000 AND 6000 -- 99% SELECT * FROM MemOptTable_With_NC_Range_Index WHERE ID BETWEEN 5000 AND 6000 -- 1%
You will see in the Execution Plan image below that Inequality Operator with Range Index Costs you only 1%, but Hash Index Costs you 99%.
So, while designing In-Memory Tables and Memory Optimized Indexes you will need to see in future that how you will be going to query that table. It also depends upon various scenarios and conditions, so always keep note of these things in advance while designing your In-Memory Tables.