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What is table Spool in SQL Server execution plan?
The Table Spool operator is one of the four spool operators that SQL Server supports. It retains a copy of all data it reads in a worktable (in tempdb) and can then later return extra copies of these rows without having to call its child operators to produce them again.
What is table spool lazy spool?
SQL Server Table Spool Operator (Lazy Spool) Te SQL Server Lazy Spool is used to build a temporary table on the TempDB and fill it in lazy manner. In other words, it fills the table by reading and storing the data only when individual rows are required by the parent operator.
How do I get an execution plan for a query in SQL Server?
To display the estimated execution plan for a query
- On the toolbar, click Database Engine Query.
- Enter the query for which you would like to display the estimated execution plan.
- On the Query menu, click Display Estimated Execution Plan or click the Display Estimated Execution Plan toolbar button.
What is a spool table?
Spooling doesn’t have anything to do with your temporary tables. Spooling is effectively internal temporary tables used by the query planner.
What is spool operator?
The spool operator creates a temporary data storage in tempdb that exists only for the duration of the current statement. This storage is populated with a subset of data that the server needs to access more than once. The idea is to make the query more efficient by “caching” the data.
What is a hash match in SQL Server?
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 index spool eager spool?
An Eager Index Spool reads all rows from its child operator into an indexed worktable, before it starts returning rows to its parent operator. In some respects, an eager index spool is the ultimate missing index suggestion, but it is not reported as such.
What is eager spool in SQL Server?
The role of the Eager Spool is to catch all the rows received from another operator and store these rows in TempDB. The word “eager” means that the operator will read ALL rows from the previously operator at one time. In other words, it will take the entire input, storing each row received.
What is a hash join in execution plan?
Figure 1. Sample Execution Plan for Hash Join Each row is inserted into a hash bucket according to the hash value computed for the hash key, so building the hash table needs memory. If the entire build input is smaller than the available memory, all rows can be inserted into the hash table.
What is seek and scan in SQL?
Explanation. An index scan or table scan is when SQL Server has to scan the data or index pages to find the appropriate records. A scan is the opposite of a seek, where a seek uses the index to pinpoint the records that are needed to satisfy the query.
What is Clustered Index Scan?
We can say, a Clustered Index Scan is same like a Table Scan operation i.e. entire index is traversed row by row to return the data set. If the SQL Server optimizer determines there are so many rows need to be returned it is quicker to scan all rows than to use index keys.
Which operator is used for enabling fine grained run time control over the query plans executed by the query engine?
The eddy operator provides an example of fine-grained run-time control by tuple routing through operators.
How do I find my plan handle?
To retrieve a snapshot of all query plans residing in the plan cache, retrieve the plan handles of all query plans in the cache by querying the sys. dm_exec_cached_plans dynamic management view. The plan handles are stored in the plan_handle column of sys. dm_exec_cached_plans .
How many DMVs are there in SQL Server?
There are two types of dynamic management views and functions: Server-scoped dynamic management views and functions.
How to understand SQL Server table spool operator?
To understand SQL Server Table Spool Operator, let us consider following query as an example. For each SalesOrderID from the SalesOrderDetails table the sub-query returns the average value of all OrderQuantity and after that average is compared with principal query to filter each SalesOrderID’s with values higher than their average.
Why is spool operator displayed three times in execution plan?
As can be seen, Spool operator displayed three times in execution plan but that doesn’t mean that three temporary tables are being created in memory that to say, all the spools are using same temporary table and this can be verified if we look at the operators hints displayed in execution plan.
How does a table spool work?
If a Table Spool is called by a Top operator, it passes a count of the number of rows stored to that Top operator. This information exchange is not in any way visible in the execution plan itself. This is typically used for a Top operator that has its Is Percent property set to True. The Table Spool will be an eager builder in this case.
What is the difference between row count spool and window spool?
The Row Count Spool operator is optimized for specific cases where the rows to be returned are empty. And the Window Spool operator is used to support the ROWS and RANGE specifications of windowing functions.