Create read replicas

MySQL  |  PostgreSQL  |  SQL Server

This page describes how to create a read replica for a Cloud SQL instance.

A read replica is a copy of the primary instance that reflects changes to theprimary in almost real time, in normal circumstances. You can use a read replicato offload read requests or analytics traffic from the primary instance.

Additionally, for disaster recovery, you can perform a regional migration. Ifa replica is across-region replica, you can perform a failover to another region;specifically, you can promote a replica to a standalone instance(in which case, existing replicas would not consider that instance as primary).

For more information about how replication works, seeReplication in Cloud SQL.

Before you begin

If you are creating the first replica for this instance, ensure that theinstance meets the requirements for primary instances.Learn more.

Create a read replica

The steps for creating a read replica are below.

Note:If you specify PostgreSQL 16 and later as the database version for your replica, then the default Cloud SQL edition is Enterprise Plus. If you either don't specify a database version or you specify a version other than PostgreSQL 16 and later, then the default Cloud SQL edition is Enterprise.

If the primary instance has aprivate IP connection and the instance accepts database connections fromCloud SQL Language Connectors only, then you can't create read replicas for the instance.

Console

  1. In the Google Cloud console, go to theCloud SQL Instances page.

    Go to Cloud SQL Instances

  2. Find the instance for which you want to create a replica, and then open themore actions menu next to the listing.
  3. SelectCreate read replica.

    If you don't see that choice, then the instance is a replica; you can't create a replica of a replica.

  4. In theCustomize your instance section of the page, update settings for your replica. Begin by clickingShow configuration options to display the groups of settings. Then, expand the desired groups to review and customize the settings. ASummary of all the options you select appears on the right. Customizing these settings is optional. Defaults are assigned in every case where no customizations are made.

    For more details about each setting, see theAbout instance settings page.

    For example, to allow other Google Cloud services, such as BigQuery, to access data in Cloud SQL and make queries against this data over an internal connection, expand theConnections group, and then deselect thePublic IP check box.

  5. ClickCreate replica.

    Cloud SQL creates a backup, if needed, and creates the replica. You are returned to the instance page for the primary.

gcloud

Create the replica:

gcloudsqlinstancescreateREPLICA_NAME\--master-instance-name=MASTER_INSTANCE_NAME

You can specify a different tier size using the--tier parameter, if needed. Also, if you're creating a replica from a primary instance, and the Cloud SQL edition for the instance is either Enterprise or is Enterprise Plus, version 16 and later, then you don't have to specify a value for this parameter. The replica inherits the machine type from the primary instance.

You can specify a different region using the--region parameter.

If the primary instance has an internal IP address only, and you want to allow other Google Cloud services, such as BigQuery, to access data in Cloud SQL and make queries against this data over an internal connection, then add the--enable-google-private-path parameter to the command.

You must create the replica in the same VPC networkas the primary instance. Youcan also specify anallocated-ip-range-name in thatVPC network. If no range is specified, the replica iscreated in a random range.

Terraform

To create a read replica, use aTerraform resource.

resource "google_sql_database_instance" "read_replica" {  name                 = "postgres-replica-instance-name"  master_instance_name = google_sql_database_instance.primary.name  region               = "europe-west4"  database_version     = "POSTGRES_14"  settings {    tier              = "db-custom-2-7680"    availability_type = "ZONAL"    disk_size         = "100"  }  # set `deletion_protection` to true, will ensure that one cannot accidentally delete this instance by  # use of Terraform whereas `deletion_protection_enabled` flag protects this instance at the GCP level.  deletion_protection = false}

REST v1

Use theinsert method of the instances resource to create the read replica. The region and databaseVersion properties must be the same as the master.

Before using any of the request data, make the following replacements:

  • project-id: The project ID
  • database-version: Enum version string (for example, POSTGRES_12)
  • primary-instance-name: The name of the primary instance
  • primary-instance-region: The region of the primary instance
  • replica-region: The region of the replica instance
  • replica-name: The name of the replica instance
  • machine-type: Enum string of the machine type. For example: "db-custom-1-3840"
  • private-network: The authorized network that you're adding or selecting to create a private connection.
You set the value of theipv4Enabled parameter tofalse because you want the Cloud SQL instance to be associated with a private IP address, not a public IP one.

HTTP method and URL:

POST https://sqladmin.googleapis.com/v1/projects/project-id/instances

Request JSON body:

{  "masterInstanceName": "primary-instance-name",  "project": "project-id",  "databaseVersion": "database-version",  "name": "replica-name",  "region": "replica-region",  "settings":  {    "tier": "machine-type",    "settingsVersion": 0,    "ipConfiguration": {    object (IpConfiguration)  },  {  "ipv4Enabled": false,  "privateNetwork":private-network,  "requireSsl": boolean,  "authorizedNetworks": [    {      object (AclEntry)    }  ],  "allocatedIpRange": string    }  }}

To send your request, expand one of these options:

curl (Linux, macOS, or Cloud Shell)

Note: The following command assumes that you have logged in to thegcloud CLI with your user account by runninggcloud init orgcloud auth login , or by usingCloud Shell, which automatically logs you into thegcloud CLI . You can check the currently active account by runninggcloud auth list.

Save the request body in a file namedrequest.json, and execute the following command:

curl -X POST \
-H "Authorization: Bearer $(gcloud auth print-access-token)" \
-H "Content-Type: application/json; charset=utf-8" \
-d @request.json \
"https://sqladmin.googleapis.com/v1/projects/project-id/instances"

PowerShell (Windows)

Note: The following command assumes that you have logged in to thegcloud CLI with your user account by runninggcloud init orgcloud auth login . You can check the currently active account by runninggcloud auth list.

Save the request body in a file namedrequest.json, and execute the following command:

$cred = gcloud auth print-access-token
$headers = @{ "Authorization" = "Bearer $cred" }

Invoke-WebRequest `
-Method POST `
-Headers $headers `
-ContentType: "application/json; charset=utf-8" `
-InFile request.json `
-Uri "https://sqladmin.googleapis.com/v1/projects/project-id/instances" | Select-Object -Expand Content

You should receive a JSON response similar to the following:

Response

{  "kind": "sql#operation",  "targetLink": "https://sqladmin.googleapis.com/v1/projects/project-id/instances/instance-id",  "status": "PENDING",  "user": "user@example.com",  "insertTime": "2020-01-21T22:43:37.981Z",  "operationType": "CREATE_REPLICA",  "name": "operation-id",  "targetId": "instance-id",  "selfLink": "https://sqladmin.googleapis.com/v1/projects/project-id/operations/operation-id",  "targetProject": "project-id"}

REST v1beta4

Use theinsert method of the instances resource to create the read replica. The region and databaseVersion properties must be the same as the master.

Before using any of the request data, make the following replacements:

  • project-id: The project ID
  • database-version: Enum version string (for example, POSTGRES_12)
  • primary-instance-name: The name of the primary instance
  • primary-instance-region: The region of the primary instance
  • replica-region: The region of the replica instance
  • replica-name: The name of the replica instance
  • machine-type: Enum string of the machine type. For example: "db-custom-1-3840"
  • private-network: The authorized network that you're adding or selecting to create a private connection.
You set the value of theipv4Enabled parameter tofalse because you want the Cloud SQL instance to be associated with a private IP address, not a public IP one.

HTTP method and URL:

POST https://sqladmin.googleapis.com/sql/v1beta4/projects/project-id/instances

Request JSON body:

{  "masterInstanceName": "primary-instance-name",  "project": "project-id",  "databaseVersion": "database-version",  "name": "replica-name",  "region": "replica-region",  "settings":  {    "tier": "machine-type",    "settingsVersion": 0,        "ipConfiguration": {    object (IpConfiguration)  },  {  "ipv4Enabled": false,  "privateNetwork":private-network,  "requireSsl": boolean,  "authorizedNetworks": [    {      object (AclEntry)    }  ],  "allocatedIpRange": string    }      }}

To send your request, expand one of these options:

curl (Linux, macOS, or Cloud Shell)

Note: The following command assumes that you have logged in to thegcloud CLI with your user account by runninggcloud init orgcloud auth login , or by usingCloud Shell, which automatically logs you into thegcloud CLI . You can check the currently active account by runninggcloud auth list.

Save the request body in a file namedrequest.json, and execute the following command:

curl -X POST \
-H "Authorization: Bearer $(gcloud auth print-access-token)" \
-H "Content-Type: application/json; charset=utf-8" \
-d @request.json \
"https://sqladmin.googleapis.com/sql/v1beta4/projects/project-id/instances"

PowerShell (Windows)

Note: The following command assumes that you have logged in to thegcloud CLI with your user account by runninggcloud init orgcloud auth login . You can check the currently active account by runninggcloud auth list.

Save the request body in a file namedrequest.json, and execute the following command:

$cred = gcloud auth print-access-token
$headers = @{ "Authorization" = "Bearer $cred" }

Invoke-WebRequest `
-Method POST `
-Headers $headers `
-ContentType: "application/json; charset=utf-8" `
-InFile request.json `
-Uri "https://sqladmin.googleapis.com/sql/v1beta4/projects/project-id/instances" | Select-Object -Expand Content

You should receive a JSON response similar to the following:

Response

{  "kind": "sql#operation",  "targetLink": "https://sqladmin.googleapis.com/sql/v1beta4/projects/project-id/instances/instance-id",  "status": "PENDING",  "user": "user@example.com",  "insertTime": "2020-01-21T22:43:37.981Z",  "operationType": "CREATE_REPLICA",  "name": "operation-id",  "targetId": "instance-id",  "selfLink": "https://sqladmin.googleapis.com/sql/v1beta4/projects/project-id/operations/operation-id",  "targetProject": "project-id"}

Create a read replica of an instance with Private Service Connect enabled

To create a read replica of an instance withPrivate Service Connect enabled, usegcloud CLI or the API. You can create this replica in either the same region or in a different region from the primary instance (cross-region read replica).

The read replica can't replicate from an instance with a different connectivity type. For example, an instance with Private Service Connect enabled can replicate only from another Private Service Connect instance. It also can't replicate from an instance that supports external IP connections or from an instance that's configured withprivate services access.

gcloud

To create a read replica of an instance, use thegcloud sql instances create command:

gcloudsqlinstancescreateREPLICA_INSTANCE_NAME\--master-instance-name=PRIMARY_INSTANCE_NAME\--project=PROJECT_ID\--region=REGION_NAME\--enable-private-service-connect\--allowed-psc-projects=ALLOWED_PROJECTS\--availability-type=AVAILABILITY_TYPE\--no-assign-ip

Make the following replacements:

  • REPLICA_INSTANCE_NAME: the name of the replica instance.
  • PRIMARY_INSTANCE_NAME: the name of the primary instance.
  • PROJECT_ID: the ID orproject number of the Google Cloud project that contains the instance.
  • REGION_NAME: the region name for the replica instance.
  • ALLOWED_PROJECTS: a list of allowed project IDs or numbers, separated by commas. If a project isn't contained in this list, then you can't use it to create an instance and enable Private Service Connect for it.

    Cloud SQL doesn't copy the allowed projects for the primary instance over to the replica. For each replica, you must create aPrivate Service Connect endpoint. If you're using theCloud SQL Auth Proxy orCloud SQL Language Connectors, then create aDNS zone andDNS record for the replicas.

  • AVAILABILITY_TYPE: enable high availability for the instance. For this parameter, specify one of the following values:
    • REGIONAL: enable high availability and is recommended for production instances. The instance fails over to another zone within your selected region.
    • ZONAL: provide no failover capability. This is the default value.

    For more information about setting and removing high availability for instances, seeConfigure an existing instance for high availability andDeactivate high availability for an instance.

REST v1

Before using any of the request data, make the following replacements:

  • PRIMARY_INSTANCE_NAME: the name of the primary instance.
  • PROJECT_ID: the ID orproject number of the Google Cloud project that contains the instance.
  • REPLICA_INSTANCE_NAME: the name of the replica instance.
  • REGION_NAME: the region name for the replica instance.
  • MACHINE_TYPE: the machine type for the instance.
  • AVAILABILITY_TYPE: enable high availability for the instance. For this parameter, specify one of the following values:
    • REGIONAL: enable high availability and is recommended for production instances. The instance fails over to another zone within your selected region.
    • ZONAL: provide no failover capability. This is the default value.

    For more information about setting and removing high availability for instances, seeConfigure an existing instance for high availability andDeactivate high availability for an instance.

  • ALLOWED_PROJECTS: a list of allowed project IDs or numbers, separated by commas. If a project isn't contained in this list, then you can't use it to create an instance and enable Private Service Connect for it.

    Cloud SQL doesn't copy the allowed projects for the primary instance over to the replica. For each replica, you must create aPrivate Service Connect endpoint. If you're using theCloud SQL Auth proxy orCloud SQL Language Connectors, then you must create aDNS zone andDNS record for the replicas.

HTTP method and URL:

POST https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/instances

Request JSON body:

{  "masterInstanceName": "PRIMARY_INSTANCE_NAME",  "project": "PROJECT_ID",  "databaseVersion": "POSTGRES_13",  "name": "REPLICA_INSTANCE_NAME",  "region": "REGION_NAME",  "kind": "sql#instance",  "settings":  {    "tier": "MACHINE_TYPE",    "availabilityType": "AVAILABILITY_TYPE",    "settingsVersion": 0,    "ipConfiguration": {      "ipv4Enabled": false,      "pscConfig": {        "allowedConsumerProjects": [ALLOWED_PROJECTS],        "pscEnabled": true      }    },    "kind": "sql#settings",    "pricingPlan": "PER_USE",    "replicationType": "ASYNCHRONOUS",    "tier": "MACHINE_TYPE"  }}

To send your request, expand one of these options:

curl (Linux, macOS, or Cloud Shell)

Note: The following command assumes that you have logged in to thegcloud CLI with your user account by runninggcloud init orgcloud auth login , or by usingCloud Shell, which automatically logs you into thegcloud CLI . You can check the currently active account by runninggcloud auth list.

Save the request body in a file namedrequest.json, and execute the following command:

curl -X POST \
-H "Authorization: Bearer $(gcloud auth print-access-token)" \
-H "Content-Type: application/json; charset=utf-8" \
-d @request.json \
"https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/instances"

PowerShell (Windows)

Note: The following command assumes that you have logged in to thegcloud CLI with your user account by runninggcloud init orgcloud auth login . You can check the currently active account by runninggcloud auth list.

Save the request body in a file namedrequest.json, and execute the following command:

$cred = gcloud auth print-access-token
$headers = @{ "Authorization" = "Bearer $cred" }

Invoke-WebRequest `
-Method POST `
-Headers $headers `
-ContentType: "application/json; charset=utf-8" `
-InFile request.json `
-Uri "https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/instances" | Select-Object -Expand Content

You should receive a JSON response similar to the following:

{  "kind": "sql#operation",  "targetLink": "https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/instances/REPLICA_INSTANCE_NAME",  "status": "PENDING",  "user": "user@example.com",  "insertTime": "2020-01-16T02:32:12.281Z",  "operationType": "CREATE_REPLICA",  "name": "OPERATION_ID",  "targetId": "REPLICA_INSTANCE_NAME",  "selfLink": "https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/operations/OPERATION_ID",  "targetProject": "PROJECT_ID"}

REST v1beta4

Before using any of the request data, make the following replacements:

  • PRIMARY_INSTANCE_NAME: the name of the primary instance.
  • PROJECT_ID: the ID orproject number of the Google Cloud project that contains the instance.
  • REPLICA_INSTANCE_NAME: the name of the replica instance.
  • REGION_NAME: the region name for the replica instance.
  • MACHINE_TYPE: the machine type for the instance.
  • AVAILABILITY_TYPE: enable high availability for the instance. For this parameter, specify one of the following values:
    • REGIONAL: enable high availability and is recommended for production instances. The instance fails over to another zone within your selected region.
    • ZONAL: provide no failover capability. This is the default value.

    For more information about setting and removing high availability for instances, seeConfigure an existing instance for high availability andDeactivate high availability for an instance.

  • ALLOWED_PROJECTS: a list of allowed project IDs or numbers, separated by commas. If a project isn't contained in this list, then you can't use it to create an instance and enable Private Service Connect for it.

    Cloud SQL doesn't copy the allowed projects for the primary instance over to the replica. For each replica, you must create aPrivate Service Connect endpoint. If you're using theCloud SQL Auth proxy orCloud SQL Language Connectors, then you must create aDNS zone andDNS record for the replicas.

HTTP method and URL:

PATCH https://sqladmin.googleapis.com/sql/v1beta4/projects/PROJECT_ID/instances

Request JSON body:

{  "masterInstanceName": "PRIMARY_INSTANCE_NAME",  "project": "PROJECT_ID",  "databaseVersion": "POSTGRES_13",  "name": "REPLICA_INSTANCE_NAME",  "region": "REGION_NAME",  "kind": "sql#instance",  "settings":  {    "tier": "MACHINE_TYPE",    "availabilityType": "AVAILABILITY_TYPE",    "settingsVersion": 0,    "ipConfiguration": {      "ipv4Enabled": false,      "pscConfig": {        "allowedConsumerProjects": [ALLOWED_PROJECTS],          "pscEnabled": true      }    },    "kind": "sql#settings",    "pricingPlan": "PER_USE",    "replicationType": "ASYNCHRONOUS",    "tier": "MACHINE_TYPE"  }}

To send your request, expand one of these options:

curl (Linux, macOS, or Cloud Shell)

Note: The following command assumes that you have logged in to thegcloud CLI with your user account by runninggcloud init orgcloud auth login , or by usingCloud Shell, which automatically logs you into thegcloud CLI . You can check the currently active account by runninggcloud auth list.

Save the request body in a file namedrequest.json, and execute the following command:

curl -X PATCH \
-H "Authorization: Bearer $(gcloud auth print-access-token)" \
-H "Content-Type: application/json; charset=utf-8" \
-d @request.json \
"https://sqladmin.googleapis.com/sql/v1beta4/projects/PROJECT_ID/instances"

PowerShell (Windows)

Note: The following command assumes that you have logged in to thegcloud CLI with your user account by runninggcloud init orgcloud auth login . You can check the currently active account by runninggcloud auth list.

Save the request body in a file namedrequest.json, and execute the following command:

$cred = gcloud auth print-access-token
$headers = @{ "Authorization" = "Bearer $cred" }

Invoke-WebRequest `
-Method PATCH `
-Headers $headers `
-ContentType: "application/json; charset=utf-8" `
-InFile request.json `
-Uri "https://sqladmin.googleapis.com/sql/v1beta4/projects/PROJECT_ID/instances" | Select-Object -Expand Content

You should receive a JSON response similar to the following:

{  "kind": "sql#operation",  "targetLink": "https://sqladmin.googleapis.com/sql/v1beta4/projects/PROJECT_ID/instances/REPLICA_INSTANCE_NAME",  "status": "PENDING",  "user": "user@example.com",  "insertTime": "2020-01-16T02:32:12.281Z",  "operationType": "CREATE_REPLICA",  "name": "OPERATION_ID",  "targetId": "REPLICA_INSTANCE_NAME",  "selfLink": "https://sqladmin.googleapis.com/sql/v1beta4/projects/PROJECT_ID/operations/OPERATION_ID",  "targetProject": "PROJECT_ID"}

Configure read replicas for IAM database authentication

If you enable thecloudsql.iam_authentication flagon the primary instance,Cloud SQL for PostgreSQL enables it on the read replicas automatically. However, if you don'tenable this flag on the primary instance, Cloud SQL for PostgreSQL doesn't enable it on the readreplicas. You can't use the replicas for IAM database authentication.

To configure a read replica for IAM database authentication:

  1. In the Google Cloud console, go to theCloud SQL Instances page.

    Go to Cloud SQL Instances

  2. To open theOverview page of an instance, click the instance name.
  3. In the Configuration tile, look for thecloudsql.iam_authentication flag. If the flag isn't on the list, enabling the flag in the read replica is unnecessary. If the flag is on the list, you must enable the flag on the read replica. If you need to enable the flag on the read replica, continue with the next step.
  4. SelectReplicas from the SQL navigation menu.
  5. Click the name of the replica you want to edit.
  6. ClickEdit.
  7. In theConfiguration options section, expandFlags.
  8. Select+ Add item.
  9. Entercloudsql.iam_authentication for the flag name. Make sure thatOn is selected for this flag.
  10. ClickSave.

Create cascading replicas

This section describes how to create and manage cascading replicas.

For information about how cascading replicas work, seeCascading replicas.

Steps to create a cascading replica

Console

  1. In the Google Cloud console, go to theCloud SQL Instances page.

    Go to Cloud SQL Instances

  2. Click theReplicas tab for the replica that will act as a parent for the replica that you want to create.
  3. ClickCreate replica.
  4. On theCreate read replica page, update the instance ID, and any other configuration options, including the name, region, and zone.
  5. ClickCreate.

    Cloud SQL creates a replica. You're returned to the instance page for the parent replica.

  6. Follow steps 4-6 for each new cascading replica that you want to create.

gcloud

  1. Create the new replica by specifying your primary replica as the primary instance using the--master-instance-name flag:
  2. gcloudsqlinstancescreateREPLICA_NAME\--master-instance-name=PARENT_REPLICA_NAME\
    Replace the following:
    • REPLICA_NAME: the unique ID for the replica that you are creating
    • PARENT_REPLICA_NAME: the name of the parent replica
  3. After you create the cascading replica, you can see that the changesmade to the primary instance are replicated through all the replicas inthe cascading replicas chain.

curl

  1. To create a replica under the parent replica, edit the following JSON code sample, and save it to a file calledrequest.json:
    {"masterInstanceName":"PARENT_REPLICA_NAME","project":"PROJECT_ID","name":"REPLICA_NAME","region":"REPLICA_REGION","settings":{"tier":"MACHINE_TYPE",}}
  2. Run the following command:
    curl-XPOST-H"Authorization: Bearer "$(gcloudauthprint-access-token)-H"Content-Type: application/json; charset=utf-8"-d@request.json"https://sqladmin.googleapis.com/v1/projects/PROJECT_ID/instances"

Troubleshoot

IssueTroubleshooting
Read replica didn't start replicating on creation.There's probably a more specific error in the log files.Inspect the logs in Cloud Logging to find the actual error.
Unable to create read replica - invalidFlagValue error.One of the flags in the request is invalid. It could be a flag you provided explicitly or one that was set to a default value.

First, check that the value of themax_connections flag is greater than or equal to the value on the primary.

If themax_connections flag is set appropriately,inspect the logs in Cloud Logging to find the actual error.

Unable to create read replica - unknown error.There's probably a more specific error in the log files.Inspect the logs in Cloud Logging to find the actual error.

If the error is:set Service Networking service account as servicenetworking.serviceAgent role on consumer project, then disable and re-enable theService Networking API. This action creates the service account necessary to continue with the process.

Disk is full.The primary instance disk size can become full during replica creation. Edit the primary instance to upgrade it to a larger disk size.
Disk space increases significantly.A slot that's not actively used to track data causes PostgreSQL tohold onto WAL segments indefinitely, causing the disk space to grow indefinitely. If you usethelogical replication and decoding features in Cloud SQL, replication slots are created and dropped automatically. Unused replication slots can be detected by querying thepg_replication_slotssystem view and filtering on theactive column. Unusedslots can be dropped to remove WAL segments using thepg_drop_replication_slot command.
The replica instance is using too much memory.The replica uses temporary memory to cache often-requested read operations, which can lead it to use more memory than the primary instance.

Restart the replica instance to reclaim the temporary memory space.

Replication stopped.The maximum storage limit was reached and automatic storage increase isn't enabled.

Edit the instance to enableautomatic storage increase.

Replication lag is consistently high.The write load is too high for the replica to handle. Replication lag takes place when the SQL thread on a replica is unable to keep up with the IO thread. Some kinds of queries or workloads can cause temporary or permanent high replication lag for a given schema. Some of the typical causes of replication lag are:
  • Slow queries on the replica. Find and fix them.
  • All tables must have a unique/primary key. Every update on such a table without a unique/primary key causes full table scans on th replica.
  • Queries likeDELETE ... WHERE field < 50000000 cause replication lag with row-based replication since a huge number of updates are piled up on the replica.

Some possible solutions include:

  • Edit the instance to increase the size of the replica.
  • Reduce the load on the database.
  • Send read traffic to the read replica.
  • Index the tables.
  • Identify and fix slow write queries.
  • Recreate the replica.
Errors when rebuilding indexes in PostgreSQL 9.6.You get an error from PostgreSQL informing you that you need to rebuild a particular index. This can be done only on the primary instance. If you create a new replica instance, you soon get the same error again.Hash indexes are not propagated to replicas in PostgreSQL versions below 10.

If you must use hash indexes, upgrade to PostgreSQL 10+. Otherwise, if you also want to use replicas, don't use hash indexes in PostgreSQL 9.6.

Query on the primary instance is always running.After creating a replica, the querySELECT * from pg_stat_activity where state = 'active' and pid = XXXX and username = 'cloudsqlreplica' is expected to run continuously on your primary instance.
Replica creation fails with timeout.Long-running uncommitted transactions on the primary instance can cause read replica creation to fail.

Recreate the replica after stopping all running queries.

If the primary instance and the replica havedifferent vCPU sizes, then there might be query performance issues because the query optimizer takes vCPU sizes into account.

To resolve this issue, complete the following steps:

  1. Turn on thelog_duration flag and set thelog_statement parameter toddl. This provides you with both the queries and the run time on the database. However, depending on your workload, this might cause performance issues.
  2. On both the primary instance and the read replica, runexplain analyze for the queries.
  3. Compare the query plan and check for differences.

If this is a specific query, then modify the query. For example, you can change the order of the joins to see if you get better performance.

What's next

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Last updated 2025-07-16 UTC.