Contents
What is hash match in SQL Server execution plan?
SQL Server Hash Match Aggregate operator is used to process the large tables that are not sorted using an index. It builds a hash table in the memory, calculates a hash value for each record, then scan all other records for that hash key.
What is a hash match?
The Hash Match represents the building of a hash table of computed hash values from each row in the input. From this MSDN article, this is the behavior of how that hash table is built. For any joins, use the first (top) input to build the hash table and the second (bottom) input to probe the hash table. …
How does a hash match work in SQL?
With a Hash match, you can expect to see the HASH: () and RESIDUAL: () predicates in an execution plan and potentially a probe row. This process works as follows: For any joins, use the first (top) input to build the hash table and the second (bottom) input to probe the hash table. Output matches (or non-matches) as dictated by the join type.
Why is a hash match used for the execution plan?
Under many circumstances, this is O (n). Of course, if your data is really large or if some values repeat many times, then the hash join will have memory issues or hash collisions — and the complexity of the algorithm increases. The key point, though, is that the optimizer chooses the algorithm that it considers best suited to the data.
How does the hashing function in SQL Server work?
The hashing function determines which bucket an element goes into. For example, you can take a row from a table, hash it into a hash value, then store the hash value into a hash table.”
How does the hash join work in SQL Server?
The hash join first reads one of the inputs and hashes the join column and puts the resulting hash and the column values into a hash table built up in memory. Then it reads all the rows in the second input, hashes those and checks the rows in the resulting hash bucket for the joining rows.”