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SQL Server 2012 : Query Plans (part 2) - Query Plan Operators

2/19/2014 7:51:36 PM
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Query Plan Operators

Join Operators

Join operators enable SQL Server to find matching rows between two tables. Prior to SQL Server 2005, there was only a single join type, the nested loop join, but since then additional join types have been added, and SQL Server now provides the three join types described in Table 1. These join types handle rows from two tables; for a self-join, the inputs may be different sets of rows from the same table.

TABLE 1: SQL Server Join Types

Nested loop Good for small tables where there is an index on the inner table on the join key
Merge join Good for medium-size tables where there are ordered indexes, or where the output needs to be ordered
Hash join Good for medium to large tables. Works well with parallel plans, and scales well.

Nested Loop

The nested loop join is the original SQL Server join type. The behavior of a nested loop is to scan all the rows in one table (the outer table) and for each row in that table, it then scans every row in the other table (the inner table). If the rows in the outer and inner tables match, then the row is included in the results.

The performance of this join is directly proportional to the number of rows in each table. It performs well when there are relatively few rows in one of the tables, which would be chosen as the inner table, and more rows in the other table, which would be used as the outer table. If both tables have a relatively large number of rows, then this join starts to take a very long time.


The merge join needs its inputs to be sorted, so ideally the tables should be indexed on the join column. Then the operator iterates through rows from both tables at the same time, working down the rows, looking for matches. Because the inputs are ordered, this enables the join to proceed quickly, and to end as soon as any range is satisfied.


The hash join operates in two phases. During the first phase, known as the build phase, the smaller of the two tables is scanned and the rows are placed into a hash table that is ideally stored in memory; but for very large tables, it can be written to disk. When every row in the build input table is hashed, the second phase starts. During the second phase, known as the probe phase, rows from the larger of the two tables are compared to the contents of the hash table, using the same hashing algorithm that was used to create the build table hash. Any matching rows are passed to the output.

The hash join has variations on this processing that can deal with very large tables, so the hash join is the join of choice for very large input tables, especially when running on multiprocessor systems where parallel plans are allowed.

Hash warnings are SQL Profiler events that are generated when hash recursion, or hash bailout, occurs. Hash recursion happens when the output from the hash operation doesn’t fit entirely in memory. Hash bailout occurs when hash recursion reaches its maximum level of recursion, and a new plan has to be chosen.
Anytime you see hash warnings, it is a potential indicator of performance problems and should be investigated.
Possible solutions to hash warnings include the following:
  • Increase memory on the server.
  • Make sure statistics exist on the join columns.
  • Make sure statistics are current.
  • Force a different type of join.

Spool Operators

The various spool operators are used to create a temporary copy of rows from the input stream and deliver them to the output stream. Spools typically sit between two other operators: The one on the right is the child, and provides the input stream. The operator on the left is the parent, and consumes the output stream.

The following list provides a brief description of each of the physical spool operators. These are the operators that actually execute. You may also see references to logical operators, which represent an earlier stage in the optimization process; these are subsequently converted to physical operators before executing the plan. The logical spool operators are Eager Spool, and Lazy Spool.

  • Index spool — This operator reads rows from the child table, places them in tempdb, and creates a nonclustered index on them before continuing. This enables the parent to take advantage of seeking against the nonclustered index on the data in tempdb when the underlying table has no applicable indexes.
  • Row count spool — This operator reads rows from the child table and counts the rows. The rows are also returned to the parent, but without any data. This enables the parent to determine whether rows exist in order to satisfy an EXISTS or NOT EXISTS requirement.
  • Table spool — This operator reads the rows from the child table and writes them into tempdb. All rows from the child are read and placed in tempdb before the parent can start processing rows.
  • Window spool — This operator expands each row into the set of rows that represent the window associated with it. It’s both a physical and logical operator.

Scan and Seek Operators

These operators enable SQL Server to retrieve rows from tables and indexes when a larger number of rows is required. This behavior contrasts with the individual row access operators key lookup and RID lookup, which are discussed in the next section.

  • Scan operator — The scan operator scans all the rows in the table looking for matching rows. When the number of matching rows is >20 percent of the table, scan can start to outperform seek due to the additional cost of traversing the index to reach each row for the seek.
There are scan operator variants for a clustered index scan, a nonclustered index scan, and a table scan.
  • Seek operator — The seek operator uses the index to find matching rows; this can be either a single value, a small set of values, or a range of values. When the query needs only a relatively small set of rows, seek is significantly faster than scan to find matching rows. However, when the number of rows returned exceeds 20 percent of the table, the cost of seek will approach that of scan; and when nearly the whole table is required, scan will perform better than seek.
There are seek operator variants for a clustered index seek and a nonclustered index seek.

Lookup Operators

Lookup operators perform the task of finding a single row of data. The following is a list of common operators:

  • Bookmark lookup — Bookmark lookup is seen only in SQL Server 2000 and earlier. It’s the way that SQL Server looks up a row using a clustered index. In SQL Server 2012 this is done using either Clustered Index Seek, RID lookup, or Key Lookup.
  • Key lookup — Key lookup is how a single row is returned when the table has a clustered index. In contrast with dealing with a heap, the lookup is done using the clustering key. The key lookup operator was added in SQL Server 2005 SP2. Prior to this, and currently when viewing the plan in text or XML format, the operator is shown as a clustered index seek with the keyword lookup.
  • RID lookup — RID lookup is how a single row is looked up in a heap. RID refers to the internal unique row identifier (hence RID), which is used to look up the row.
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