Troubleshooting Common Performance Problems with Memory-Optimized Hash Indexes
Applies To: Azure SQL Database, SQL Server 2016 Preview
This topic will focus on troubleshooting and working around common issues with hash indexes.
Search Requires a Subset of Hash Index Key Columns
Issue: Hash indexes require values for all index key columns in order to compute the hash value, and locate the corresponding rows in the hash table. Therefore, if a query includes equality predicates for only a subset of the index keys in the WHERE clause, SQL Server cannot use an index seek to locate the rows corresponding to the predicates in the WHERE clause.
In contrast, ordered indexes like the disk-based nonclustered indexes and the memory-optimized nonclustered indexes support index seek on a subset of the index key columns, as long as they are the leading columns in the index.
Symptom: This results in a performance degradation, as SQL Server needs to execute full table scans rather than an index seek, which is typically a faster operation.
How to troubleshoot: Besides the performance degradation, inspection of the query plans will show a scan instead of an index seek. If the query is fairly simple, inspection of the query text and index definition will also show whether the search requires a subset of the index key columns.
Consider the following table and query:
CREATE TABLE [dbo].[od] ( o_id INT NOT NULL, od_id INT NOT NULL, p_id INT NOT NULL, CONSTRAINT PK_od PRIMARY KEY NONCLUSTERED HASH (o_id, od_id) WITH (BUCKET_COUNT = 10000) ) WITH (MEMORY_OPTIMIZED = ON) SELECT p_id FROM dbo.od WHERE o_id=1
The table has a hash index on the two columns (o_id, od_id), while the query has an equality predicate on (o_id). As the query has equality predicates on only a subset of the index key columns, SQL Server cannot perform an index seek operation using PK_od; instead, SQL Server has to revert to a full index scan.
Workarounds: There are a number of possible workarounds. For example:
Recreate the index as type nonclustered instead of nonclustered hash. The memory-optimized nonclustered index is ordered, and thus SQL Server can perform an index seek on the leading index key columns. The resulting primary key definition for the example would be constraint PK_od primary key nonclustered.
Change the current index key to match the columns in the WHERE clause.
Add a new hash index that matches with the columns in the WHERE clause of the query. In the example, the resulting table definition would look at follows:
CREATE TABLE dbo.od ( o_id INT NOT NULL, od_id INT NOT NULL, p_id INT NOT NULL, CONSTRAINT PK_od PRIMARY KEY NONCLUSTERED HASH (o_id,od_id) WITH (BUCKET_COUNT=10000), INDEX ix_o_id NONCLUSTERED HASH (o_id) WITH (BUCKET_COUNT=10000) ) WITH (MEMORY_OPTIMIZED=ON)
Note that a memory-optimized hash index does not perform optimally if there are a lot of duplicate rows for a given index key value: in the example, if the number of unique values for the column o_id is much smaller than the number of rows in the table, it would not be optimal to add an index on (o_id); instead, changing the type of the index PK_od from hash to nonclustered would be the better solution. For more information, see Determining the Correct Bucket Count for Hash Indexes.