These statistics are useful for understanding and tuning the bucket counts. It can also be used to detect cases where the index key has many duplicates.
A large average chain length indicates that many rows are hashed to the same bucket. This could happen because:
If the number of empty buckets is low or the average and maximum chain lengths are similar, it is likely that the total bucket count is too low. This causes many different index keys to hash to the same bucket.
If the number of empty buckets is high or the maximum chain length is high relative to the average chain length, it is likely that there are many rows with duplicate index key values or there is a skew in the key values. All rows with the same index key value hash to the same bucket, hence there is a long chain length in that bucket.
Long chain lengths can significantly impact the performance of all DML operations on individual rows, including SELECT and INSERT. Short chain lengths along with a high empty bucket count are in indication of a bucket_count that is too high. This decreases the performance of index scans.
sys.dm_db_xtp_hash_index_stats scans the entire table. So, if there are large tables in your database, sys.dm_db_xtp_hash_index_stats may take a long time run.
For more information, see Hash Indexes for Memory-Optimized Tables.
|object_id||int||The object ID of parent table.|
|xtp_object_id||bigint||ID of the memory-optimized table.|
|index_id||int||The index ID.|
|total_bucket_count||bigint||The total number of hash buckets in the index.|
|empty_bucket_count||bigint||The number of empty hash buckets in the index.|
|avg_chain_length||bigint||The average length of the row chains over all the hash buckets in the index.|
|max_chain_length||bigint||The maximum length of the row chains in the hash buckets.|
|xtp_object_id||bigint||The in-memory OLTP object ID that corresponds to the memory-optimized table.|
Requires VIEW DATABASE STATE permission on the server.
A. Troubleshooting hash index bucket count
The following query can be used to troubleshoot the hash index bucket count of an existing table. The query returns statistics about percentage of empty buckets and chain length for all hash indexes on user tables.
SELECT QUOTENAME(SCHEMA_NAME(t.schema_id)) + N'.' + QUOTENAME(OBJECT_NAME(h.object_id)) as [table], i.name as [index], h.total_bucket_count, h.empty_bucket_count, FLOOR(( CAST(h.empty_bucket_count as float) / h.total_bucket_count) * 100) as [empty_bucket_percent], h.avg_chain_length, h.max_chain_length FROM sys.dm_db_xtp_hash_index_stats as h JOIN sys.indexes as i ON h.object_id = i.object_id AND h.index_id = i.index_id JOIN sys.memory_optimized_tables_internal_attributes ia ON h.xtp_object_id=ia.xtp_object_id JOIN sys.tables t on h.object_id=t.object_id WHERE ia.type=1 ORDER BY [table], [index];
For details on how to interpret the results of this query, see Hash Indexes for Memory-Optimized Tables.
B. Hash index statistics for internal tables
Certain features use internal tables that leverage hash indexes, for example columnstore indexes on memory-optimized tables. The following query returns stats for hash indexes on internal tables that are linked to user tables.
SELECT QUOTENAME(SCHEMA_NAME(t.schema_id)) + N'.' + QUOTENAME(OBJECT_NAME(h.object_id)) as [user_table], ia.type_desc as [internal_table_type], i.name as [index], h.total_bucket_count, h.empty_bucket_count, h.avg_chain_length, h.max_chain_length FROM sys.dm_db_xtp_hash_index_stats as h JOIN sys.indexes as i ON h.object_id = i.object_id AND h.index_id = i.index_id JOIN sys.memory_optimized_tables_internal_attributes ia ON h.xtp_object_id=ia.xtp_object_id JOIN sys.tables t on h.object_id=t.object_id WHERE ia.type!=1 ORDER BY [user_table], [internal_table_type], [index];
Note that the BUCKET_COUNT of index on internal tables cannot be changed, thus the output of this query should be considered informative only. No action is required.
This query is not expected to return any rows unless you are using a feature that leverages hash indexes on internal tables. The following memory-optimized table contains a columnstore index. After creating this table, you will see hash indexes on internal tables.
CREATE TABLE dbo.table_columnstore ( c1 INT NOT NULL PRIMARY KEY NONCLUSTERED, INDEX ix_columnstore CLUSTERED COLUMNSTORE ) WITH (MEMORY_OPTIMIZED=ON)