Execute the EXPLAIN statement

In this exercise, you look at the EXPLAIN function and how it can display the execution plan that the PostgreSQL planner generates for a supplied statement.

Before you start

You need your own Azure subscription to complete this exercise. If you don’t have an Azure subscription, you can create an Azure free trial.

Additionally, you need to have the following installed on your computer:

  • Visual Studio Code.
  • Postgres Visual Studio Code Extension by Microsoft.
  • Azure CLI.
  • Git.

Create the exercise environment

In this and later exercises, you use a Bicep script to deploy the Azure Database for PostgreSQL - Flexible Server and other resources into your Azure subscription. The Bicep scripts are located in the /Allfiles/Labs/Shared folder of the GitHub repository you cloned earlier.

Download and install Visual Studio Code and the PostgreSQL extension

If you don’t have Visual Studio Code installed:

  1. In a browser, navigate to Download Visual Studio Code and select the appropriate version for your operating system.

  2. Follow the installation instructions for your operating system.

  3. Open Visual Studio Code.

  4. From the left menu, select Extensions to display the Extensions panel.

  5. In the search bar, enter PostgreSQL. The PostgreSQL extension for Visual Studio Code icon is displayed. Make sure you select the one by Microsoft.

  6. Select Install. The extension installs.

Download and install Azure CLI and Git

If you don’t have Azure CLI or Git installed:

  1. In a browser, navigate to Install the Azure CLI and follow the instructions for your operating system.

  2. In a browser, navigate to Download and install Git and follow the instructions for your operating system.

Download the exercise files

If you already cloned the GitHub repository containing the exercise files, Skip downloading the exercise files.

To download the exercise files, you clone the GitHub repository containing the exercise files to your local machine. The repository contains all the scripts and resources you need to complete this exercise.

  1. Open Visual Studio Code if it isn’t already open.

  2. Select Show all commands (Ctrl+Shift+P) to open the command palette.

  3. In the command palette, search for Git: Clone and select it.

  4. In the command palette, enter the following to clone the GitHub repo containing exercise resources and press Enter:

     https://github.com/MicrosoftLearning/mslearn-postgresql.git

  5. Follow the prompts to select a folder to clone the repository into. The repository is cloned into a folder named mslearn-postgresql in the location you selected.

  6. When asked if you want to open the cloned repository, select Open. The repository opens in Visual Studio Code.

Deploy resources into your Azure subscription

If your Azure resources are already installed, Skip deploying resources.

This step guides you through using Azure CLI commands from Visual Studio Code to create a resource group and run a Bicep script to deploy the Azure services necessary for completing this exercise into your Azure subscription.

📝 If you are doing multiple modules in this learning path, you can share the Azure environment between them. In that case, you only need to complete this resource deployment step once.

  1. Open Visual Studio Code if it isn’t already open, and open the folder where you cloned the GitHub repository.

  2. Expand the mslearn-postgresql folder in the Explorer pane.

  3. Expand the Allfiles/Labs/Shared folder.

  4. Right-click the Allfiles/Labs/Shared folder and select Open in Integrated Terminal. This selection opens a terminal window at in the Visual Studio Code window.

  5. The terminal might open a powershell window by default. For this section of the lab, you want to use the bash shell. Besides the + icon, there’s a dropdown arrow. Select it and select Git Bash or Bash from the list of available profiles. This selection opens a new terminal window with the bash shell.

    📝 You can close the powershell terminal window if you want to, but it is not necessary. You can have multiple terminal windows open at the same time.

  6. In the terminal window, run the following command to sign-in to your Azure account:

     az login

    This command opens a new browser window prompting you to sign-in to your Azure account. After logging in, return to the terminal window.

  7. Next, you run three commands to define variables to reduce redundant typing when using Azure CLI commands to create Azure resources. The variables represent the name to assign to your resource group (RG_NAME), the Azure region (REGION) into which resources are deployed, and a randomly generated password for the PostgreSQL administrator sign-in (ADMIN_PASSWORD).

    In the first command, the region assigned to the corresponding variable is eastus, but you can also replace it with a location of your preference.

     REGION=eastus

    The following command assigns the name to be used for the resource group that houses all the resources used in this exercise. The resource group name assigned to the corresponding variable is rg-learn-work-with-postgresql-$REGION, where $REGION is the location you previously specified. However, you can change it to any other resource group name that suits your preference or that you might already have.

     RG_NAME=rg-learn-work-with-postgresql-$REGION

    The final command randomly generates a password for the PostgreSQL admin sign-in. Make sure you copy it to a safe place so that you can use it later to connect to your PostgreSQL flexible server.

     #!/bin/bash
    
 # Define array of allowed characters explicitly
 chars=( {a..z} {A..Z} {0..9} '!' '@' '#' '$' '%' '^' '&' '*' '(' ')' '_' '+' )
    
 a=()
 for ((i = 0; i < 100; i++)); do
     rand_char=${chars[$RANDOM % ${#chars[@]}]}
     a+=("$rand_char")
 done
    
 # Join first 18 characters without delimiter
 ADMIN_PASSWORD=$(IFS=; echo "${a[*]:0:18}")
    
 echo "Your randomly generated PostgreSQL admin user's password is:"
 echo "$ADMIN_PASSWORD"
 echo "Please copy it to a safe place, as you will need it later to connect to your PostgreSQL flexible server."

  8. (Skip if using your default subscription.) If you have access to more than one Azure subscription, and your default subscription isn’t the one in which you want to create the resource group and other resources for this exercise, run this command to set the appropriate subscription, replacing the <subscriptionName|subscriptionId> token with either the name or ID of the subscription you want to use:

     az account set --subscription 16b3c013-d300-468d-ac64-7eda0820b6d3

  9. (Skip if you’re using an existing resource group) Run the following Azure CLI command to create your resource group:

     az group create --name $RG_NAME --location $REGION

  10. Finally, use the Azure CLI to execute a Bicep deployment script to provision Azure resources in your resource group:

     az deployment group create --resource-group $RG_NAME --template-file "Allfiles/Labs/Shared/deploy-postgresql-server.bicep" --parameters adminLogin=pgAdmin adminLoginPassword=$ADMIN_PASSWORD

    The Bicep deployment script provisions the Azure services required to complete this exercise into your resource group. The resources deployed are an Azure Database for PostgreSQL - Flexible Server. The bicep script also creates a database - which can be configured on the commandline as a parameter.

    The deployment typically takes several minutes to complete. You can monitor it from the bash terminal or navigate to the Deployments page for the resource group you previously created and observe the deployment progress there.

  11. Since the script creates a random name for the PostgreSQL server, you can find the name of the server by running the following command:

     az postgres flexible-server list --query "[].{Name:name, ResourceGroup:resourceGroup, Location:location}" --output table

    Write down the name of the server, as you need it to connect to the server later in this exercise.

    📝 You can also find the name of the server in the Azure portal. In the Azure portal, navigate to Resource groups and select the resource group you previously created. The PostgreSQL server is listed in the resource group.

Troubleshooting deployment errors

You might encounter a few errors when running the Bicep deployment script. The most common messages and the steps to resolve them are:

  • If you previously ran the Bicep deployment script for this learning path and then deleted the resources, you might receive an error message like the following if you’re attempting to rerun the script within 48 hours of deleting the resources:

      {"code": "InvalidTemplateDeployment", "message": "The template deployment 'deploy' is not valid according to the validation procedure. The tracking id is '4e87a33d-a0ac-4aec-88d8-177b04c1d752'. See inner errors for details."}
    
  Inner Errors:
  {"code": "FlagMustBeSetForRestore", "message": "An existing resource with ID '/subscriptions/{subscriptionId}/resourceGroups/rg-learn-postgresql-ai-eastus/providers/Microsoft.CognitiveServices/accounts/{accountName}' has been soft-deleted. To restore the resource, you must specify 'restore' to be 'true' in the property. If you don't want to restore existing resource, please purge it first."}

    If you receive this message, modify the previous azure deployment group create command to set the restore parameter equal to true and rerun it.

  • If the selected region is restricted from provisioning specific resources, you must set the REGION variable to a different location and rerun the commands to create the resource group and run the Bicep deployment script.

      {"status":"Failed","error":{"code":"DeploymentFailed","target":"/subscriptions/{subscriptionId}/resourceGroups/{resourceGrouName}/providers/Microsoft.Resources/deployments/{deploymentName}","message":"At least one resource deployment operation failed. Please list deployment operations for details. Please see https://aka.ms/arm-deployment-operations for usage details.","details":[{"code":"ResourceDeploymentFailure","target":"/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.DBforPostgreSQL/flexibleServers/{serverName}","message":"The resource write operation failed to complete successfully, because it reached terminal provisioning state 'Failed'.","details":[{"code":"RegionIsOfferRestricted","message":"Subscriptions are restricted from provisioning in this region. Please choose a different region. For exceptions to this rule please open a support request with Issue type of 'Service and subscription limits'. See https://review.learn.microsoft.com/en-us/azure/postgresql/flexible-server/how-to-request-quota-increase for more details."}]}]}}

  • If the lab requires AI resources, you might get the following error. This error occurs when the script is unable to create an AI resource due to the requirement to accept the responsible AI agreement. If that is the case, use the Azure portal user interface to create an Azure AI Services resource, and then rerun the deployment script.

      {"code": "InvalidTemplateDeployment", "message": "The template deployment 'deploy' is not valid according to the validation procedure. The tracking id is 'f8412edb-6386-4192-a22f-43557a51ea5f'. See inner errors for details."}
     
  Inner Errors:
  {"code": "ResourceKindRequireAcceptTerms", "message": "This subscription cannot create TextAnalytics until you agree to Responsible AI terms for this resource. You can agree to Responsible AI terms by creating a resource through the Azure Portal then trying again. For more detail go to https://go.microsoft.com/fwlink/?linkid=2164190"}

Connect to the PostgreSQL extension in Visual Studio Code

In this section, you connect to the PostgreSQL server using the PostgreSQL extension in Visual Studio Code. You use the PostgreSQL extension to run SQL scripts against the PostgreSQL server.

  1. Open Visual Studio Code if it isn’t already opened and open the folder where you cloned the GitHub repository.

  2. Select the PostgreSQL icon in the left menu.

    📝 If you do not see the PostgreSQL icon, select the Extensions icon and search for PostgreSQL. Select the PostgreSQL extension by Microsoft and select Install.

  3. If you already created a connection to your PostgreSQL server, skip to the next step. To create a new connection:

    1. In the PostgreSQL extension, select + Add Connection to add a new connection.

    2. In the NEW CONNECTION dialog box, enter the following information:

      • Server name: <your-server-name>.postgres.database.azure.com
      • Authentication type: Password
      • User name: pgAdmin
      • Password: The random password you previously generated.
      • Check the Save password checkbox.
      • Connection name: <your-server-name>

    3. Test the connection by selecting Test Connection. If the connection is successful, select Save & Connect to save the connection, otherwise review the connection information, and try again.

  4. If not already connected, select Connect for your PostgreSQL server. You’re connected to the Azure Database for PostgreSQL server.

  5. Expand the Server node and its databases. The existing databases are listed.

  6. If you didn’t create the zoodb database already, select File, Open file and navigate to the folder where you saved the scripts. Select ../Allfiles/Labs/02/Lab2_ZooDb.sql and Open.

  7. On the lower right of Visual Studio Code, make sure the connection is green. If it isn’t, it should say PGSQL Disconnected. Select the PGSQL Disconnected text and then select your PostgreSQL server connection from the list in the command palette. If it asks for a password, enter the password you previously generated.

  8. Time to create the database.

    1. Highlight the DROP and CREATE statements and run them.

    2. If you highlight just the SELECT current_database() statement and run it, you notice that the database is currently set to postgres. You need to change it to zoodb.

    3. Select the ellipsis in the menu bar with the run icon and select Change PostgreSQL Database. Select zoodb from the list of databases.

      📝 You can also change the database on the query pane. You can note the server name and database name under the query tab itself. Selecting the database name will show a list of databases. Select the zoodb database from the list.

    4. Run the SELECT current_database() statement again to confirm that the database is now set to zoodb.

    5. Highlight the Create tables, Create foreign keys, and Populate tables sections and run them.

    6. Highlight the 3 SELECT statements at the end of the script and run them to verify that the tables were created and populated.

Practice EXPLAIN ANALYZE

In this section, you run the EXPLAIN statement to analyze the execution plan of a query. The EXPLAIN statement provides information about how PostgreSQL executes a query, including the estimated cost and the actual time taken to execute the query.

  1. In the Visual Studio Code window, select File, Open File, and then navigate to the lab scripts. Select ../Allfiles/Labs/03/Lab3_RepopulateZoo.sql and then select Open. If necessary, reconnect to the server by selecting the PGSQL Disconnected text and then selecting your PostgreSQL server connection from the list in the command palette. If it asks for a password, enter the password you previously generated.

  2. On the lower right of Visual Studio Code, make sure the connection is green. If it isn’t, it should say PGSQL Disconnected. Select the PGSQL Disconnected text and then select your PostgreSQL server connection from the list in the command palette. If it asks for a password, enter the password you previously generated.

  3. Run the SELECT current_database() statement to check your current database. Verify if the connection is currently set to the zoodb database. If it isn’t, you can change the database to zoodb. To change the database, select the ellipsis in the menu bar with the run icon and selecting Change PostgreSQL Database. Select zoodb from the list of databases. Verify that the database is now set to zoodb by running the SELECT current_database(); statement.

  4. Select Run to execute the query. This query repopulates the zoodb database.

  5. In the Visual Studio Code window, select File, Open File, and then navigate to the lab scripts. Select ../Allfiles/Labs/03/Lab3_explain.sql and then select Open. If necessary, reconnect to the server by selecting the PGSQL Disconnected text and then selecting your PostgreSQL server connection from the list in the command palette. If it asks for a password, enter the password you previously generated.

  6. On the lower right of Visual Studio Code, make sure the connection is green. If it isn’t, it should say PGSQL Disconnected. Select the PGSQL Disconnected text and then select your PostgreSQL server connection from the list in the command palette. If it asks for a password, enter the password you previously generated.

  7. Run the SELECT current_database() statement to check your current database. Verify if the connection is currently set to the zoodb database. If it isn’t, you can change the database to zoodb. To change the database, select the ellipsis in the menu bar with the run icon and selecting Change PostgreSQL Database. Select zoodb from the list of databases. Verify that the database is now set to zoodb by running the SELECT current_database(); statement.

  8. In the Lab file, in the section 1. Investigate EXPLAIN ANALYZE highlight and run Statement A and Statement B separately.

    1. Which statement updated the database, and why?

    2. How many milliseconds did it take to plan Statement A?

    3. What was the execution time for Statement B?

Practice EXPLAIN

In this section, you run the EXPLAIN statement to analyze the execution plan of a query. The EXPLAIN statement provides information about how PostgreSQL executes a query, including the estimated cost and the actual time taken to execute the query.

  1. In the Lab file, in the section 2. Investigate EXPLAIN highlight and run that statement.

    What sort key was used, and why?

  2. In the Lab file, in the section 3. Investigate EXPLAIN options highlight and run each statement separately. Compare the query plan statistics for each option.

Clean-Up

  1. If you don’t need this PostgreSQL server anymore for other exercises, to avoid incurring unnecessary Azure costs, delete the resource group created in this exercise.

  2. If you want to keep the PostgreSQL server running, you can leave it running. If you don’t want to leave it running, you can stop the server to avoid incurring unnecessary costs in the bash terminal. To stop the server, run the following command:

     az postgres flexible-server stop --name <your-server-name> --resource-group $RG_NAME

    Replace <your-server-name> with the name of your PostgreSQL server.

    📝 You can also stop the server from the Azure portal. In the Azure portal, navigate to Resource groups and select the resource group you previously created. Select the PostgreSQL server and then select Stop from the menu.

  3. If needed, delete the git repository you cloned earlier.

You successfully completed this exercise. You learned how to use the EXPLAIN statement to analyze the execution plan of a query. Additionally, you learned how to use the EXPLAIN ANALYZE statement to analyze the execution plan of a query with actual execution statistics.