Clustered Index Merge

Introduction

The Clustered Index Merge operator is used when rows in a clustered index need to be changed, and the required changes can be a combination of inserted, updates, and deletes. Unlike the specialized operators (Clustered Index Insert, Clustered Index Update, and Clustered Index Delete) that can simultaneously modify data in one or more nonclustered indexes as well, the Clustered Index Merge can only target a single index, which has to be the clustered index. The rows to be affected, as well as the required action and the new values of all columns in the index, are produced by the operator’s child subtree. After performing a modification, the operator returns data from that row to its parent; this data can be both the original or the new data.

The Clustered Index Merge operator is almost identical in functionality to the Clustered Index Update operator when it has its Action Column property set.

A Clustered Index Merge operator that targets a clustered columnstore index is represented in SSMS (and most other tools) as a Columnstore Index Merge operator in the graphical execution plan. To match the representation of the execution plan that most people use, I’ll focus the descriptions below on how Clustered Index Merge operates when the Storage property is ‘RowStore’, and call out relevant differences on the separate Columnstore Index Merge page.

Visual appearance in execution plans

Depending on the tool being used, a Clustered Index Merge operator is displayed in a graphical execution plan as shown below:

SQL Server Management Studio

Azure Data Studio

Plan Explorer

(version 17.4 and up)

(until version 17.3)

SSMS icon for Clustered Index Merge Old version of SSMS icon for Clustered Index Merge

(no icon)

Icon for Clustered Index Merge in PlanExplorer

Algorithm

The basic algorithm for the Clustered Index Merge operator is as shown below:

Flowchart for the logic of the Clustered Index Merge operatorNote that this flowchart is a simplification. It doesn’t show that execution stops whenever a row is returned, and resumes where it was upon next being called.

Acquire locks

Before modifying data, the Clustered Index Merge operator will always acquire the necessary locks (or verify that it already has them). Which locks exactly are taken depends on the isolation level of the transaction, as well as the action column (if any). It is never possible to completely eliminate all locking on data modifications.

This operator does not release the locks it takes, nor does any other operator in the same execution plan. Locks for data modifications are always held until the end of the transaction.

A full discussion of locking strategies used for the various transaction isolation levels is beyond the scope of this website.

Action Column

The Clustered Index Merge operator uses the input columns listed in its Action Column property to specify, per individual row, what type of modification is needed (insert, update, or delete).

The known values and their meaning for the action column (for any operator, not just for Clustered Index Merge) are:

  • 1: Update
  • 3: Delete
  • 4: Insert

It is currently unknown whether there are more possible values.

If the Action Column property is not provided, then all input rows will be processed as updates to an existing row.

Insert row

When the action column indicates that a row needs to be inserted, the Clustered Index Merge operator uses the Predicate property to find the values for all columns in that row. The Clustered Index Merge operator does not actively check any constraints. Primary keys and unique constraints are implicitly checked because the storage engine throws an error as soon as data is written to a unique index that would cause a duplicate entry. For other constraints, the optimizer adds additional operators to gather relevant data, then checks the validity of the data in an Assert operator.

Update row

When the action column indicates that a row needs to be updated, the Clustered Index Merge operator uses the values provided for the index key columns in the input to locate the row. If then uses the Predicate property to find the new values to assign to the data in that row. The Clustered Index Merge always does an update in place; the row will never move to another location in the index because the index columns are not changed. If a query can change the indexed columns of a clustered index, then the optimizer uses a Split operator to convert these updates to equivalent insert and delete operations (optionally combined with a Collapse operator for better performance).

The optimizer ensures that foreign key and check constraints are checked by other operators in the execution plan. An Assert operator is typically used to force a rollback of the operation if a constraint would be violated.

Delete row

When the action column indicates that a row needs to be deleted, the Clustered Index Merge operator uses the values provided for the index key columns in the input to locate the row, and then deletes it from the index. The Predicate property is ignored for rows that have to be deleted. No constraints are checked by the operator; the optimizer ensures that foreign key and check constraints are checked by other operators in the execution plan. An Assert operator is typically used to force a rollback of the operation if a constraint would be violated.

Return row

Like any operator, Clustered Index Merge is invoked by calling its GetNext() method. And like any other operator, it responds by returning a row, with the columns listed in the Output List property. These columns can be values from the input data, new values of the columns in the clustered index, or old (before update) values of those columns. The latter can be recognized in the execution plan by their name: the same name as the column, with “_OLD” appended.

The output rows can be used by parent operators for additional logic, such as constraint checking or propagating the changes to other indexes, indexed views, etc.

If the parent operator does not need any data, the Output List property is empty and an empty row is returned for each row deleted.

Operator properties

The properties below are specific to the Clustered Index Merge, or have a specific meaning when appearing on it. For all other properties, see Common properties. Properties that are included on the Common properties page but are also included below for their specific meaning for the Clustered Index Merge operator are marked with a *.

Property nameDescription
ActionColumnThis property lists the name of the column from the input that determines for each input row whether it updates index data in place, inserts new data in the index, or deletes an existing row from the index.
DMLRequestSortWhen set to true, the modification might qualify for minimal logging if additional conditions apply. Details of those additional conditions and the requirements for DMLRequestSort to be true can be found here.
ObjectThis property indicates the clustered index that the Clustered Index Merge operator will modify, using four part naming (database, schema, table, index).
The subproperties of the Object property represent the four name parts separately, but also include two additional properties:
  • Index Kind: Represents what kind of index data is targeted. For a Clustered Index Merge, this is always Clustered.
  • Storage: Determines the storage type of the index. For a Clustered Index Merge, this can be either RowStore or ColumnStore, but note that most tools represent the operator as a Columnstore Index Merge if the Storage property is ColumnStore.
PartitionedWhen this property is present and set to true, the target of the Clustered Index Merge is a partitioned index.
PredicateMaps columns from the input stream to the columns in the index indicated by the Object property, or sets these columns to hardcoded values or variables from the query text.
For input rows that are to be processed as a delete (as determined by the data in the column listed in the Action Column property), only the columns that are part of the index key are used. (Plus the uniqueifier column if the index is not declared as unique).

Implicit properties

This table below lists the behavior of the implicit properties for the Clustered Index Merge operator.

Property nameDescription
Batch Mode enabledThe Clustered Index Merge operator supports row mode execution only.
BlockingThe Clustered Index Merge operator is non-blocking.
Memory requirementThe Clustered Index Merge operator does not have any special memory requirement.
Order-preservingThe Clustered Index Merge operator is fully order-preserving.
Parallelism awareThe Clustered Index Merge operator does not support parallelism. It can only be used in a serial plan, or in a serial section of a parallel plan.
Segment awareThe Clustered Index Merge operator is not segment aware.

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