Ashwani Roy Understanding Graphical Execution Plans Level 200
• Query Processing lifecycle by Database Engine
• Elements in a Execution Plans
• Important Execution Plan Operators
Agenda
• Logical and Physical Operators • Parallelism Physical Operators• Cursor Operators• Language Elements
Operators in an Execution Plan
Columns in a Plan
Rows EstimateIO
Executes EstimateCPU
StmtId AvgRowSize
NodeId TotalSubtreeCost
Parent OutputList
PhysicalOp Warnings
LogicalOp Type
Argument Parallel
DefinedValues EstimateExecutions
EstimateRows
Important Operators in Execution Plans
Select (Result)
Sort Clustered Index Seek
Clustered Index Scan
Non-clustered Index Scan
Non-clustered Index Seek
Table Scan RID Lookup Key Lookup Hash Match
Nested Loops
Merge Join Top Compute Scalar
Constant Scan
Filter Lazy Spool Spool Eager Spool Stream Aggregate
Distribute Streams
Repartition Streams
Gather Streams
Bitmap Split
Index Seek
• Reads B-tree entries to determine the data page• The Argument column contains the name of the
nonclustered index being used• Prefered for highly selective queries
Index Scan
• Horizontal traversal of the leaf level of the index from the first page to the last
• Retrieves all rows from the nonclustered index• The Argument column contains the name of the
nonclustered index being used
Clustered Index Scan
• The clustered index scan’s logical and physical operator scans the clustered index
• The Argument column contains the name of the clustered index
• If the table does not have Clustered Index the same Query will produce Table Scan
Clustered Index Seek
• Cluster index seek • Uses the seeking ability of indexes to retrieve rows• The Argument column contains the name of the
clustered index being used• Seek() predicate contains the columns used for
seeking
Bookmark Lookups
• Uses a bookmark to look up a row in a clustered index or table
• The Argument column contains the bookmark label
• Can be removed by covering columns• May have a performance improvement
KEY LOOKUP
• A Key Lookup is a bookmark lookup on a table with a clustered index.
• Means that the optimizer cannot retrieve the rows in a single operation, and has to use a clustered key (or a row ID) to return the corresponding rows from a clustered index (or from the table itself).
• Performance can be improved by making Non-Clustered Index or Covering Index
RID Lookup
• A type of bookmark lookup
• Occurs on a heap table (a table that doesn't have a clustered index)
• Uses a row identifier to find the rows to return.
Nested Loop
• The top input to the nested loop is the outer table• The bottom input to the nested loop is the inner table• For each outer row, searches for matching rows are in
the inner input table• Effective if the outer input is very small and the inner
input is preindexed and very large• Optimizer sometimes sorts the outer input to improve
locality of the searches on the index over the inner input
• Best when search exploits an index (indexes on join columns are used)
• Low memory requirement
Hash Join
• The top input is build input, the smaller of the two inputs
• The bottom input is probe input• The hash join first scans or computes the whole
build input• Requires at least one equality clause in the join
predicate• Good for ad-hoc queries
Merge Join
• Both inputs should be sorted on the merge column keys
• An index on a correct set of columns is useful• A many-to-many merge join uses a temporary
table to store rows• Very fast if the data that you want can be
obtained presorted from existing B-tree indexes
WHICH JOIN IS GOOD
• NONE AND ALL• A Merge Join is an efficient way to join two tables,#
• when the join columns are pre sorted • if the join columns are not pre sorted, the query
optimizer has the option of• a) sorting the join columns first, then performing
a Merge Join, or • b) performing a less efficient Hash Join. The
query optimizer considers all the options and generally chooses the execution plan that uses the least resources.
Stream Aggregation
• The argument column of the plan output shows the list of columns of the GROUP BY or DISTINCT clause
• The list of aggregate expressions will appear in the Defined Values column of the plan output
• Best for smaller sets or sets already sorted• Input is sorted and output is ordered
Hash Aggregation
• Used with large sets• Aggregations are evaluated while building the
hash• Input can be in random order; output is always
in random order