Querying FINAL Tables In ClickHouse: Excluding Deleted Rows

by Alex Johnson 60 views

ClickHouse, a powerful open-source column-oriented database management system, is renowned for its speed and efficiency in handling large datasets. However, its approach to deletes can sometimes pose a challenge. Understanding how ClickHouse processes deletes and how to query the final state of a table is crucial for ensuring data consistency in your applications. This article delves into the intricacies of querying the FINAL version of a ClickHouse table to exclude deleted rows, providing practical examples and best practices.

Understanding ClickHouse and Deletes

ClickHouse's architecture treats deletes as a heavy operation. When a delete command is issued, the rows are not immediately physically removed from the storage. Instead, they are marked for deletion, and the actual removal happens asynchronously in the background, during a process called merges and mutations. This approach optimizes write performance but can lead to inconsistencies if queries are not designed to account for these marked-for-deletion rows. Consequently, querying a table might return rows that are technically marked as deleted but haven't yet been physically purged.

For many use cases, especially those requiring real-time analytics or reporting, it's essential to query the "final" state of a table. This means obtaining results that exclude any rows marked for deletion, as if the delete operations have already been fully processed. Ensuring this behavior is vital for maintaining data integrity and providing accurate insights.

To effectively query the final state of a ClickHouse table, it is imperative to understand how ClickHouse handles data mutations and merges. Data in ClickHouse is stored in immutable parts, and updates or deletes are performed by creating new parts that reflect the changes. These parts are then merged in the background. When querying the final state, ClickHouse needs to consider these merges and mutations to provide an accurate view of the data.

Querying the FINAL Table in ClickHouse

To ensure that your queries against ClickHouse tables return the final state, effectively excluding rows marked for deletion, you can leverage the FINAL modifier in your SQL queries. This modifier instructs ClickHouse to consider the results of any background merges and mutations, providing a consistent view of the data.

The FINAL modifier is a powerful tool that tells ClickHouse to consider the final state of the data, effectively excluding any rows that are marked for deletion but haven't been physically removed yet. This is particularly useful in scenarios where data consistency is paramount, and you need to ensure that your queries reflect the most up-to-date information.

Using the FINAL Modifier

The simplest way to query the final state of a table is to append the FINAL keyword to your SELECT statement. Here’s how you can do it:

SELECT * FROM my_table FINAL;

This query ensures that the results returned do not include any rows that have been marked for deletion. ClickHouse will internally handle the process of filtering out these rows, providing you with a clean and consistent dataset.

Practical Examples

Consider a scenario where you have a table named user_activity that tracks user interactions on a platform. If you frequently delete old activity records to manage storage, querying the final state becomes crucial for accurate reporting.

SELECT
    user_id,
    COUNT(*) AS total_activities
FROM user_activity FINAL
GROUP BY user_id
ORDER BY total_activities DESC;

This query calculates the total activities for each user, excluding any deleted records. The FINAL modifier ensures that the counts are accurate, reflecting the current state of the data.

Another common use case is when you need to generate reports based on the latest data. For instance, you might want to track the number of new users added each day, excluding any users who have been deleted.

SELECT
    event_date,
    COUNT(*) AS new_users
FROM user_registration FINAL
WHERE event_type = 'registration'
GROUP BY event_date
ORDER BY event_date;

By using the FINAL modifier, you can ensure that your reports accurately reflect the number of active users, providing valuable insights into user growth and engagement.

Querying with Models (e.g., Laravel)

When working with frameworks like Laravel, you often interact with databases through models. To query the final version of a table using a model, you need to ensure that the FINAL modifier is applied in your queries.

Using DB::connection() in Laravel

If you are using Laravel’s database connection directly, you can append the FINAL modifier to your SQL query. For example:

$results = DB::connection('clickhouse')
    ->select('SELECT * FROM my_table FINAL');

This approach allows you to leverage the raw SQL capabilities of ClickHouse while still benefiting from Laravel’s database connection management.

Integrating with Eloquent Models

To integrate the FINAL modifier with Eloquent models, you can extend the query builder to include the FINAL keyword. Here’s how you can achieve this:

First, create a custom query builder class that extends Laravel’s default query builder:

namespace App\QueryBuilders;

use Illuminate\Database\Query\Builder;

class ClickHouseQueryBuilder extends Builder
{
    public function final()
    {
        return $this->statement = str_replace('FROM', 'FINAL FROM', $this->toSql());
    }

    public function toSql()
    {
        if ($this->statement) {
            return $this->statement;
        }

        return parent::toSql();
    }

    public function get($columns = ['*'])
    {
        $this->statement = str_replace('FROM', 'FINAL FROM', parent::toSql());
        return parent::get($columns);
    }
}

Next, create a custom connection class that extends Laravel’s default connection and overrides the query builder:

namespace App\Connections;

use Illuminate\Database\ClickHouseConnection as BaseClickHouseConnection;
use App\QueryBuilders\ClickHouseQueryBuilder;
use Illuminate\Database\Query\Processors\ClickHouseProcessor;
use Illuminate\Database\Query\Grammars\ClickHouseGrammar;

class ClickHouseConnection extends BaseClickHouseConnection
{
    /**
     * Get a new query builder instance.
     *
     * @return \App\QueryBuilders\ClickHouseQueryBuilder
     */
    public function query()
    {
        return new ClickHouseQueryBuilder(
            $this,
            $this->getQueryGrammar(),
            $this->getPostProcessor()
        );
    }

    /**
     * Get the default post processor instance.
     *
     * @return \Illuminate\Database\Query\Processors\ClickHouseProcessor
     */
    protected function getDefaultPostProcessor()
    {
        return new ClickHouseProcessor();
    }

    /**
     * Get the default query grammar instance.
     *
     * @return \Illuminate\Database\Query\Grammars\ClickHouseGrammar
     */
    protected function getDefaultQueryGrammar()
    {
        return $this->withTablePrefix(new ClickHouseGrammar());
    }
}

Then, update your database configuration to use this custom connection:

'clickhouse' => [
    'driver' => 'clickhouse',
    'host' => env('CLICKHOUSE_HOST', 'localhost'),
    'port' => env('CLICKHOUSE_PORT', '8123'),
    'database' => env('CLICKHOUSE_DATABASE', 'default'),
    'username' => env('CLICKHOUSE_USER', 'default'),
    'password' => env('CLICKHOUSE_PASSWORD', ''),
    'options' => [
        'ssl' => false,
    ],
    'prefix' => '',
    'name' => 'clickhouse',
    'options' => [
        'database_driver' => 'ClickHouse',
    ],
    'driver_class' => App\Connections\ClickHouseConnection::class,
],

Finally, in your model, you can use the final() method to query the final state:

namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use App\QueryBuilders\ClickHouseQueryBuilder;

class MyTable extends Model
{
    protected $connection = 'clickhouse';
    protected $table = 'my_table';

    /**
     * Create a new Eloquent query builder for the model.
     *
     * @param  \Illuminate\Database\Query\Builder  $query
     * @return \App\QueryBuilders\ClickHouseQueryBuilder|static
     */
    public function newEloquentBuilder($query)
    {
        return new ClickHouseQueryBuilder($query->getQuery());
    }

    /**
     * Scope a query to only include resources from final table.
     *
     * @param  \App\QueryBuilders\ClickHouseQueryBuilder  $query
     * @return \App\QueryBuilders\ClickHouseQueryBuilder
     */
    public function scopeFinal($query)
    {
        return $query->getQuery()->final();
    }
}

Now, you can query the final state using your model like this:

$results = MyTable::final()->get();

This approach seamlessly integrates the FINAL modifier into your Eloquent model queries, ensuring that you always retrieve the final state of the table.

Addressing the PhpStorm Issue

The side note in the original question mentions an issue with PhpStorm not recognizing methods like getRows() when using DB::connection('clickhouse')->table('my_table'), even with 'fix_default_query_builder' => true set in the database config. This might be due to PhpStorm’s static analysis not fully recognizing the extended functionality provided by the ClickHouse-specific query builder.

To address this, you can use PHP DocBlocks to provide hints to PhpStorm about the available methods. For example:

/**
 * @method \ankdev\ClickHouse\Query\Builder getRows()
 */
class MyTable extends Model
{
    // ...
}

By adding this DocBlock to your model, you can help PhpStorm understand the available methods, improving code completion and reducing false warnings.

Best Practices and Considerations

While the FINAL modifier is a powerful tool for ensuring data consistency, it's essential to use it judiciously. Overusing FINAL can impact query performance, as ClickHouse needs to perform additional processing to filter out deleted rows. Therefore, consider the following best practices:

  • Use FINAL only when necessary: If you are querying data where the inclusion of deleted rows does not significantly impact the results, you might not need to use FINAL. For example, in some analytical queries, the slight discrepancy caused by including marked-for-deletion rows might be acceptable.
  • Optimize table merges: Ensure that your table merges are configured appropriately. Frequent merges can reduce the number of parts ClickHouse needs to process when querying with FINAL, improving performance.
  • Consider materialized views: For complex queries or frequently accessed datasets, consider using materialized views to precompute results. Materialized views can significantly improve query performance by storing the results of a query, which can then be queried directly.
  • Monitor query performance: Regularly monitor the performance of your queries, especially those using FINAL. Use ClickHouse’s profiling tools to identify any bottlenecks and optimize your queries accordingly.

Conclusion

Querying the FINAL version of a table in ClickHouse is crucial for ensuring data consistency and accuracy, especially in scenarios where deletes are frequent. By using the FINAL modifier in your SQL queries and integrating it with your models, you can effectively exclude deleted rows from your results. However, it’s essential to use FINAL judiciously and consider its performance implications. By following the best practices outlined in this article, you can leverage the power of ClickHouse while maintaining data integrity.

For more in-depth information about ClickHouse and its features, refer to the official ClickHouse documentation. This resource provides comprehensive details on various aspects of ClickHouse, including table engines, query optimization, and best practices for data management.