Hash Key Generator Sql Server With Set Output
- Hash In Sql
- Hash Key Generator Sql Server With Set Output In Excel
- Hash Key Generator Sql Server With Set Output List
- Sql Server Hash Join
Prerequisite
Sep 20, 2006 output all rows in the hash table. Before executing a query with a hash aggregate, SQL Server uses cardinality estimates to estimate how much memory we need to execute the query. With a hash join, we store each build row, so the total memory requirement is proportional to the number and size of the build rows. Set nocount on. If you use a GUID, you'll have to create your own mechanism to capture the last inserted value (i.e. Retrieve the GUID prior to insertion or use the SQL Server 2005 OUTPUT clause). Now that we understand how to auto generate key values and what they look like, let's examine the storage impacts of each approach. Dec 23, 2018 Let's see the usage of the MS SQL function HASHBYTES witch purpose is to hash values. MS SQL function HASHBYTES was introduced in MS SQL version 2005 supporting MD2, MD4, MD5, SHA, SHA1 hashing algorithms. From MS SQL server version 2012 additionally the SHA2256, SHA2512 algorithms were introduced. Using hash values in SSIS to determine when to insert or update rows. By: Koen Verbeeck Updated. The source rows with the destination rows using the business key. If a match is found - an update - the surrogate key and the hash are retrieved. If you use SQL Server 2012. The output conforms to the algorithm standard: 128 bits (16 bytes) for MD2, MD4, and MD5; 160 bits (20 bytes) for SHA and SHA1; 256 bits (32 bytes) for SHA2256, and 512 bits (64 bytes) for SHA2512. Applies to: SQL Server 2012 (11.x) and later.
Important context information for understanding this article is available at:
Hash In Sql
Practical numbers
MD5 hashes are also used to ensure the data integrity of files. Because the MD5 hash algorithm always produces the same output for the same given input, users can compare a hash of the source file with a newly created hash of the destination file to check that it is intact and unmodified. An MD5 hash is NOT encryption. Apr 14, 2013 While most output is similar under the Messages tab, we now receive the Results table with a record set describing the Index Key, related key from the Clustered Index (or RID if supported by a Heap table), as well as a Unique Qualifier if we have not specified the index key to be unique because for SQL Server to lock the resource this is required.
When creating a hash index for a memory-optimized table, the number of buckets needs to be specified at create time. In most cases the bucket count would ideally be between 1 and 2 times the number of distinct values in the index key.
However, even if the BUCKET_COUNT is moderately below or above the preferred range, the performance of your hash index is likely to be tolerable or acceptable.At minimum, consider giving your hash index a BUCKET_COUNT roughly equal to the number of rows you predict your memory-optimized table will grow to have.
Suppose your growing table has 2,000,000 rows, but the prediction is it will grow 10 times to 20,000,000 rows. Start with a bucket count that is 10 times the number of rows in the table. This gives you room for an increased quantity of rows.
- Ideally you would increase the bucket count when the quantity of rows reaches the initial bucket count.
- Even if the quantity of rows grows to 5 times larger than the bucket count, the performance is still good in most situations.
Suppose a hash index has 10,000,000 distinct key values.
- A bucket count of 2,000,000 would be about as low as you could accept. The degree of performance degradation could be tolerable.
Too many duplicate values in the index?
If the hash indexed values have a high rate of duplicates, the hash buckets suffer longer chains.
Assume you have the same SupportEvent table from the earlier T-SQL syntax code block. The following T-SQL code demonstrates how you can find and display the ratio of all values to unique values:
- A ratio of 10.0 or higher means a hash would be a poor type of index. Consider using a nonclustered index instead,
Troubleshooting hash index bucket count
This section discusses how to troubleshoot the bucket count for your hash index.
Monitor statistics for chains and empty buckets
You can monitor the statistical health of your hash indexes by running the following T-SQL SELECT. The SELECT uses the data management view (DMV) named sys.dm_db_xtp_hash_index_stats.
Compare the SELECT results to the following statistical guidelines:
- Empty buckets:
- 33% is a good target value, but a larger percentage (even 90%) is usually fine.
- When the bucket count equals the number of distinct key values, approximately 33% of the buckets are empty.
- A value below 10% is too low.
- Chains within buckets:
- An average chain length of 1 is ideal in case there are no duplicate index key values. Chain lengths up to 10 are usually acceptable.
- If the average chain length is greater than 10, and the empty bucket percent is greater than 10%, the data has so many duplicates that a hash index might not be the most appropriate type.
Hash Key Generator Sql Server With Set Output In Excel
Demonstration of chains and empty buckets
The following T-SQL code block gives you an easy way to test a SELECT * FROM sys.dm_db_xtp_hash_index_stats;. The code block completes in 1 minute. Here are the phases of the following code block:
- Creates a memory-optimized table that has a few hash indexes.
- Populates the table with thousands of rows.
a. A modulo operator is used to configure the rate of duplicate values in the StatusCode column.
b. The loop inserts 262,144 rows in approximately 1 minute. - PRINTs a message asking you to run the earlier SELECT from sys.dm_db_xtp_hash_index_stats.
The preceding INSERT loop does the following:
- Inserts unique values for the primary key index, and for ix_OrderSequence.
- Inserts a couple hundred thousands rows which represent only 8 distinct values for
StatusCode. Therefore there is a high rate of value duplication in index ix_StatusCode.
For troubleshooting when the bucket count is not optimal, examine the following output of the SELECT from sys.dm_db_xtp_hash_index_stats. For these results we added WHERE Object_Name(h.object_id) = 'SalesOrder_Mem' to the SELECT copied from section D.1.
Our SELECT results are displayed after the code, artificially split into two narrower results tables for better display.
- Here are the results for bucket count.
| IndexName | total_bucket_count | empty_bucket_count | EmptyBucketPercent |
|---|---|---|---|
| ix_OrderSequence | 32768 | 13 | 0 |
| ix_StatusCode | 8 | 4 | 50 |
| PK_SalesOrd_B14003... | 262144 | 96525 | 36 |
- Next are the results for chain length.
| IndexName | avg_chain_length | max_chain_length |
|---|---|---|
| ix_OrderSequence | 8 | 26 |
| ix_StatusCode | 65536 | 65536 |
| PK_SalesOrd_B14003... | 1 | 8 |
Let us interpret the preceding results tables for the three hash indexes:
ix_StatusCode:
- 50% of the buckets are empty, which is good.
- However, the average chain length is very high at 65536.
- This indicates a high rate of duplicate values.
- Therefore, using a hash index is not appropriate in this case. A nonclustered index should be used instead.
ix_OrderSequence:
- 0% of the buckets are empty, which is too low.
- The average chain length is 8, even though all values in this index are unique.
- Therefore the bucket count should be increased, to reduce the average chain length closer to 2 or 3.
- Because the index key has 262144 unique values, the bucket count should be at least 262144.
- If future growth is expected, the bucket count should be higher.
Primary key index (PK_SalesOrd_...):
Hash Key Generator Sql Server With Set Output List
- 36% of the buckets are empty, which is good.
- The average chain length is 1, which is also good. No change is needed.
Balancing the trade-off
OLTP workloads focus on individual rows. Full table scans are not usually in the performance critical path for OLTP workloads. Therefore, the trade-off you must balance is between quantity of memory utilization versus performance of equality tests and insert operations.
If memory utilization is the bigger concern:
- Choose a bucket count close to the number of index key records.
- The bucket count should not be significantly lower than the number of index key values, as this impacts most DML operations as well the time it takes to recover the database after server restart.
If performance of equality tests is the bigger concern:
- A higher bucket count, of two or three times the number of unique index values, is appropriate. A higher count means:
- Faster retrievals when looking for one specific value.
- An increased memory utilization.
- An increase in the time required for a full scan of the hash index.
Additional reading
Hash Indexes for Memory-Optimized Tables
Nonclustered Indexes for Memory-Optimized Tables
Returns the SQL Server password hash of the input value that uses the current version of the password hashing algorithm.
PWDENCRYPT is an older function and might not be supported in a future release of SQL Server. Use HASHBYTES instead. HASHBYTES provides more hashing algorithms.
Syntax
Arguments
password
Is the password to be encrypted. password is sysname.
Sql Server Hash Join
Return Types
varbinary(128)
Permissions
PWDENCRYPT is available to public.
See Also
Security Functions (Transact-SQL)
PWDCOMPARE (Transact-SQL)