ALTER INDEX

Capella Operational

The ALTER INDEX statement increases or decreases the number of index replicas and partition replicas.

The ALTER VECTOR INDEX statement is a synonym for the ALTER INDEX statement. Both statements have the same functionality.

Purpose

You can use the ALTER INDEX statement to increase or decrease the number of replicas, or to drop a specified index replica temporarily. You can also use it to perform the same alterations to a partitioned index and any replica partitions. You may use this statement when you encounter any of the following situations:

An imbalance occurs due to a particular index growing faster than expected, and a replica is needed on a different node.
An imbalance occurs due to a cluster of indexes being dropped on a single node.
A node is scheduled for removal, and its index replicas need to be dropped.
The automated process of rebalancing does not give the expected results.
Other types of scaling up or scaling down are needed.

If a node goes down while an ALTER INDEX operation is happening, then the index would rollback to its original node (not affecting queries) and a notification would appear.

Prerequisites

Only users with the RBAC role of Administrator are allowed to run the ALTER INDEX directive.

Syntax

alter-index ::= 'ALTER' 'INDEX' ( index-path '.' index-name |
                index-name 'ON' keyspace-ref ) index-using? index-with

The ALTER INDEX statement provides 2 possible syntaxes for specifying the index and the keyspace where the index is located.

index-name	(Required) A unique name that identifies the index.
index-path	(Optional) One possible syntax for specifying the keyspace. See Index Path.
keyspace-ref	(Optional) The other possible syntax for specifying the keyspace. See Index Name ON Keyspace Reference.
index-using	(Optional) Specifies the index type. See USING Clause.
index-with	(Required) Specifies options for the index. See WITH Clause.

Index Path

index-path ::= keyspace-full | keyspace-prefix | keyspace-partial

You can use a dotted notation to specify the index and the keyspace on which the index is built. This syntax provides compatibility with legacy versions of Couchbase Server. The index path may be a full keyspace path, a keyspace prefix, or a keyspace partial.

If there is a hyphen (-) inside the index name or any part of the index path, you must wrap the index name or that part of the index path in backticks (` `). See the examples on this page.

Index Path: Full Keyspace

keyspace-full ::= namespace ':' bucket '.' scope '.' collection

If the index is built on a named collection, the index path may be a full keyspace path, including namespace, bucket, scope, and collection, followed by the index name. In this case, the query context is ignored.

namespace	(Required) An identifier that refers to the namespace of the keyspace. Currently, only the `default` namespace is available.
bucket	(Required) An identifier that refers to the bucket name of the keyspace.
scope	(Required) An identifier that refers to the scope name of the keyspace.
collection	(Required) An identifier that refers to the collection name of the keyspace.

For example, default:`travel-sample`.inventory.airline.`idx-name` indicates the idx-name index on the airline collection in the inventory scope in the default:`travel-sample` bucket.

Index Path: Keyspace Prefix

keyspace-prefix ::= ( namespace ':' )? bucket

If the index is built on the default collection in the default scope within a bucket, the index path may be just an optional namespace and the bucket name, followed by the index name. In this case, the query context should not be set.

namespace	(Optional) An identifier that refers to the namespace of the keyspace. Currently, only the `default` namespace is available. If the namespace name is omitted, the default namespace in the current session is used.
bucket	(Required) An identifier that refers to the bucket name of the keyspace.

For example, default:`travel-sample`.def_type indicates the def_type index on the default collection in the default scope in the default:`travel-sample` bucket.

Index Path: Keyspace Partial

keyspace-partial ::= collection

Alternatively, if the keyspace is a named collection, the index path may be just the collection name, followed by the index name. In this case, you must set the query context to indicate the required namespace, bucket, and scope.

collection

(Required) An identifier that refers to the collection name of the keyspace.

For example, airline.`idx-name` indicates the idx-name index on the airline collection, assuming that the query context is set.

Index Name ON Keyspace Reference

keyspace-ref ::= keyspace-path | keyspace-partial

You can use the index name with the ON keyword and a keyspace reference to specify the keyspace on which the index is built. The keyspace reference may be a keyspace path or a keyspace partial.

If there is a hyphen (-) inside the index name or any part of the keyspace reference, you must wrap the index name or that part of the keyspace reference in backticks (` `). See the examples on this page.

Keyspace Reference: Keyspace Path

keyspace-path ::= ( namespace ':' )? bucket ( '.' scope '.' collection )?

If the keyspace is a named collection, or the default collection in the default scope within a bucket, the keyspace reference may be a keyspace path. In this case, the query context should not be set.

namespace	(Optional) An identifier that refers to the namespace of the keyspace. Currently, only the `default` namespace is available. If the namespace name is omitted, the default namespace in the current session is used.
bucket	(Required) An identifier that refers to the bucket name of the keyspace.
scope	(Optional) An identifier that refers to the scope name of the keyspace. If omitted, the bucket’s default scope is used.
collection	(Optional) An identifier that refers to the collection name of the keyspace. If omitted, the default collection in the bucket’s default scope is used.

For example, def_type ON default:`travel-sample` indicates the def_type index on the default collection in the default scope in the default:`travel-sample` bucket.

Similarly, `idx-name` ON default:`travel-sample`.inventory.airline indicates the idx-name index on the airline collection in the inventory scope in the default:`travel-sample` bucket.

Keyspace Reference: Keyspace Partial

keyspace-partial ::= collection

Alternatively, if the keyspace is a named collection, the keyspace reference may be just the collection name. In this case, you must set the query context to indicate the required namespace, bucket, and scope.

collection

(Required) An identifier that refers to the collection name of the keyspace.

For example, `idx-name` ON airline indicates the idx-name index on the airline collection, assuming the query context is set.

USING Clause

index-using ::= 'USING' 'GSI'

The index type for a secondary index must be Global Secondary Index (GSI). The USING GSI keywords are optional and may be omitted.

WITH Clause

index-with ::= 'WITH' expr

Use the WITH clause to specify additional options.

expr	An object with the following properties.

Name Description Schema

Name	Description	Schema
action required	A string denoting the operation to be performed. The possible values are: move You cannot use this action in Couchbase Capella, as it defaults to file-based index rebalancing. replica_count Alters the number of replicas. You must use the `num_replica` property to specify the required number of replicas. The planner decides where to place any new index replicas on the available index nodes, based on the server load. In Couchbase Capella, you cannot restrict the placement of index replicas to specified nodes. drop_replica Drops a specified replica temporarily; for example, to repair a replica. You must use the `replicaId` property to specify the replica to drop.	enum (move, replica_count, drop_replica)
num_replica optional	Required if `action` is set to `replica_count`. An integer specifying the number of replicas of the index. The index service will automatically distribute these indexes amongst the index nodes in the cluster for load balancing and high availability purposes. The index service attempts to distribute the replicas based on the server groups in use in the cluster where possible. (In Couchbase Capella, you cannot restrict the placement of index replicas to specified nodes.)	Integer
nodes optional	You cannot use this property in Couchbase Capella, as it defaults to file-based index rebalancing.	String array
replicaId optional	Required if `action` is set to `drop_replica`. An integer, specifying a replica ID.	Integer

action
required

A string denoting the operation to be performed. The possible values are:

move: You cannot use this action in Couchbase Capella, as it defaults to file-based index rebalancing.
replica_count: Alters the number of replicas. You must use the num_replica property to specify the required number of replicas. The planner decides where to place any new index replicas on the available index nodes, based on the server load. In Couchbase Capella, you cannot restrict the placement of index replicas to specified nodes.
drop_replica: Drops a specified replica temporarily; for example, to repair a replica. You must use the replicaId property to specify the replica to drop.

enum (move, replica_count, drop_replica)

num_replica
optional

Required if action is set to replica_count.

An integer specifying the number of replicas of the index. The index service will automatically distribute these indexes amongst the index nodes in the cluster for load balancing and high availability purposes. The index service attempts to distribute the replicas based on the server groups in use in the cluster where possible. (In Couchbase Capella, you cannot restrict the placement of index replicas to specified nodes.)

Integer

nodes
optional

You cannot use this property in Couchbase Capella, as it defaults to file-based index rebalancing.

String array

replicaId
optional

Required if action is set to drop_replica.

An integer, specifying a replica ID.

Integer

Usage

If you attempt to alter an index which is still scheduled for background creation, the request fails.

The statement will not work while the cluster is undergoing a rebalance.

Moving an Index or Index Replicas

You cannot use this statement to move indexes or index replicas in Couchbase Capella, as it defaults to file-based index rebalancing.

Likewise, you cannot use this statement to repartition an index across a different number of nodes.

Changing the Replica Count

When changing the number of replicas, the specified number of replicas must be less than the number of index nodes available for placement. If the specified number of replicas is greater than or equal to the number of index nodes available for placement, then the operation will fail.

In Couchbase Capella, you cannot restrict the placement of index replicas to specified nodes.

When increasing the number of replicas, no single index node will host more than 1 replica of the same index, or the same partition of the same index. Replicas are distributed across the available server groups.

When reducing the number of replicas, the index service will first drop unhealthy replicas, where an unhealthy replica is a replica with missing partitions. After all unhealthy replicas are dropped, the index service will if necessary drop replicas with the highest replica ID. An unhealthy replica may not have the highest replica ID, so after an index reduction there may be gaps in the sequence of replica IDs — for example, 1, 2, 4, where replica ID 3 was dropped.

Dropping a Specific Replica

When dropping a replica, the index topology does not change. The indexing service remembers the number of partitions and replicas specified for this index. Given sufficient capacity, the dropped replica is rebuilt after the next rebalance — although it may be placed on a different index node, depending on the resource usage statistics of the available nodes.

To find the ID of an index replica and see which node it’s placed on, you can use the Indexes page in the Couchbase Capella UI or query the system:indexes catalog.

When dropping a replica, it’s possible to leave a server group with no replica. For a partitioned index, run a rebalance to move a replica into the vacant server group.

Index Redistribution

Couchbase Capella redistributes indexes automatically on rebalance. For more information, see Rebalance.

Return Value

If the statement succeeds, then:

The query returns an empty array.
The index alteration is visible in the Indexes tab.
After the alteration is complete, the new indexes begin to service query scans.

If the statement fails, then:

The original indexes continue to service query scans.
The UI Log and Query tab have the appropriate error message.

Some common errors include:

Error Message Possible Cause

Error Message	Possible Cause
`GSI index xxxxxxxx not found`	Mistyped an index name
`move index is disabled`	Attempted to move a replica
`"nodes" clause is disabled with alter index as file based rebalance (shard affinity) is enabled`	Attempted to restrict replicas to specified nodes when altering the number of replicas
`syntax error - at \",\"`	Missed a double-quote mark (`"`)
`Fail to alter index: Fail to drop replica`	Replica does not exist or was mistyped
`Unsupported action value`	Mistyped the `"action"`

GSI index xxxxxxxx not found

Mistyped an index name

move index is disabled

Attempted to move a replica

"nodes" clause is disabled with alter index as file based rebalance (shard affinity) is enabled

Attempted to restrict replicas to specified nodes when altering the number of replicas

syntax error - at \",\"

Missed a double-quote mark (")

Fail to alter index: Fail to drop replica

Replica does not exist or was mistyped

Unsupported action value

Mistyped the "action"

Examples

To try the examples in this section, set the query context to the inventory scope in the travel sample dataset. For more information, see Query Context.

The examples in this section assume that you have a cluster with at least 4 nodes. The nodes are named svc-dqi-node-001, svc-dqi-node-002, and svc-dqi-node-003, and svc-dqi-node-004. The nodes in your cluster may have different names or IP addresses.

Example 1. Increasing the number of replicas

Create an index on node svc-dqi-node-001 with replicas on nodes svc-dqi-node-002 and svc-dqi-node-003, then increase the number of replicas to 4. New replicas may be placed on any available index nodes in the cluster.

CREATE INDEX country_idx ON airport(country, city)
WITH {"nodes": ["svc-dqi-node-001:18091",
                "svc-dqi-node-002:18091",
                "svc-dqi-node-003:18091"]};

ALTER INDEX country_idx ON airport
WITH {"action": "replica_count", "num_replica": 4};

Example 2. Decreasing the number of replicas

Create an index on node svc-dqi-node-001 with replicas on nodes svc-dqi-node-002 and svc-dqi-node-003, then decrease the number of replicas to 1.

CREATE INDEX country_idx ON airport(country, city)
WITH {"nodes": ["svc-dqi-node-001:18091",
                "svc-dqi-node-002:18091",
                "svc-dqi-node-003:18091"]};

ALTER INDEX country_idx ON airport
WITH {"action": "replica_count", "num_replica": 1};

Example 3. Dropping a specific replica

Create an index with 2 replicas, and specify that nodes svc-dqi-node-001, svc-dqi-node-002, svc-dqi-node-003, and svc-dqi-node-004 should be available for index and replica placement. Then delete replica 2.

CREATE INDEX country_idx ON airport(country, city)
USING GSI
WITH {"num_replica": 2,
      "nodes": ["svc-dqi-node-001:18091",
                "svc-dqi-node-002:18091",
                "svc-dqi-node-003:18091",
                "svc-dqi-node-004:18091"]};

ALTER INDEX country_idx ON airport
WITH {"action": "drop_replica", "replicaId": 2};