ClickHouse SELECT FINAL Explained: Examples & Use Cases

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ClickHouse SELECT FINAL Explained: Examples & Use Cases

ClickHouse SELECT FINAL Explained: Examples & Use Cases

Hey guys! Ever wondered how to squeeze every last drop of performance out of your ClickHouse queries? Well, buckle up, because we’re diving deep into the SELECT FINAL clause! This often-overlooked feature can be a game-changer when dealing with CollapsingMergeTree tables, but it’s crucial to understand how it works and when to use it. Let’s break it down with clear examples and real-world use cases.

Table of Contents

What is SELECT FINAL in ClickHouse?

In ClickHouse, the SELECT FINAL clause is specifically designed to be used with MergeTree table engines, particularly those that involve data mutations or collapsing (like CollapsingMergeTree , VersionedCollapsingMergeTree , and ReplacingMergeTree ). These engines are optimized for high-speed data ingestion and often perform mutations (updates, deletes) asynchronously. This means that when you query the table normally, you might see intermediate states of the data – rows that are marked for deletion but haven’t been physically removed yet, or multiple versions of the same record.

SELECT FINAL forces ClickHouse to apply all pending mutations and collapse the data before returning the results. In essence, it shows you the final state of the data, as if all the mutations had already been fully processed. It ensures that you see the most up-to-date and consistent view of your data, reflecting all deletions, updates, and merges that are supposed to have happened. Without FINAL , you might get duplicate or outdated information, which can be problematic for reporting or decision-making.

The magic of FINAL lies in its ability to handle the asynchronous nature of mutations in MergeTree tables. When data is inserted or mutated, ClickHouse doesn’t immediately rewrite the entire table. Instead, it creates new data parts and marks old parts for deletion or merging. These merges happen in the background. A regular SELECT query simply reads the data from the available parts, without considering whether they’re marked for deletion or have been superseded by newer versions. This is where FINAL steps in. It tells ClickHouse to perform a merge on the fly, essentially consolidating all the data parts and applying the necessary transformations before returning the result. This process, while ensuring data accuracy, comes with a performance cost, so it’s important to use it judiciously.

SELECT FINAL Example: CollapsingMergeTree

Let’s imagine a scenario where we’re tracking user activity on a website using a CollapsingMergeTree . We record events like page views, button clicks, and form submissions. Each event has a UserID , a Timestamp , and a Sign column. The Sign column indicates whether the event is a start (+1) or an end (-1). This allows us to track active sessions – when a user starts interacting with the website, we insert a row with Sign = 1 , and when they stop, we insert a row with Sign = -1 . The CollapsingMergeTree engine then collapses these events, effectively summing the Sign values for each user. If the sum is 1, it means the user is currently active. If it’s 0, it means the user is inactive.

First, let’s create a table: 

CREATE TABLE user_activity (
    UserID UInt32,
    Timestamp DateTime,
    Event String,
    Sign Int8
) ENGINE = CollapsingMergeTree(Timestamp, Sign)
ORDER BY (UserID, Timestamp);

Now, let’s insert some data representing user sessions: 

INSERT INTO user_activity VALUES
(123, '2024-10-27 10:00:00', 'Page View', 1),
(123, '2024-10-27 10:05:00', 'Button Click', 1),
(123, '2024-10-27 10:10:00', 'Form Submission', 1),
(123, '2024-10-27 10:15:00', 'Form Submission', -1),
(123, '2024-10-27 10:20:00', 'Button Click', -1),
(456, '2024-10-27 11:00:00', 'Page View', 1),
(456, '2024-10-27 11:05:00', 'Button Click', 1),
(456, '2024-10-27 11:10:00', 'Button Click', -1);

If we run a simple SELECT query without FINAL : 

SELECT UserID, sum(Sign) AS ActiveSessions
FROM user_activity
GROUP BY UserID;

We might get an inaccurate result because the collapsing hasn’t happened yet. We might see UserID 123 with ActiveSessions = 1 or 2 , even though they have effectively ended some sessions. ClickHouse hasn’t had a chance to merge the +1 and -1 events.

However, if we use SELECT FINAL :

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SELECT UserID, sum(Sign) AS ActiveSessions
FROM user_activity
FINAL
GROUP BY UserID;

SELECT FINAL forces ClickHouse to collapse the data before calculating the sum. This guarantees that we get the correct ActiveSessions count for each user, reflecting the true state of their sessions. For example, UserID 123 will likely show ActiveSessions = 1 and UserID 456 might show ActiveSessions = 1 (depending on if the background merges have occurred yet). FINAL ensures the result reflects the actual active sessions at the time of the query.

When to Use SELECT FINAL

SELECT FINAL is your friend in these scenarios:

  • Reporting on CollapsingMergeTree tables: When you need accurate, real-time reports on aggregated data from CollapsingMergeTree tables, FINAL ensures that the collapsing is applied before the aggregation, preventing double-counting or inaccurate sums.
  • Data consistency is paramount: If you absolutely need to see the most up-to-date and consistent view of your data, especially when dealing with mutations, FINAL is a must-have.
  • Debugging and testing: FINAL can be useful for verifying the correctness of your collapsing logic or for debugging data inconsistencies in your MergeTree tables.

However, remember that SELECT FINAL comes with a performance overhead. It forces ClickHouse to perform a merge on the fly, which can be resource-intensive, especially on large tables. So, use it wisely!

When to Avoid SELECT FINAL

Think twice before using SELECT FINAL in these situations:

  • Performance is critical: If your queries are performance-sensitive and you can tolerate a slight delay in data consistency, avoid FINAL . The background merges will eventually catch up, and your regular SELECT queries will become more accurate over time.
  • Large datasets: On very large tables, the on-the-fly merge performed by FINAL can be extremely slow and resource-intensive. Consider alternative strategies like running OPTIMIZE TABLE…FINAL outside of query time, if possible.
  • Near real-time is good enough: If you don’t need absolute real-time accuracy and can accept a small degree of data staleness, you can often skip FINAL . The background merges will eventually converge to the correct state.

Alternatives to SELECT FINAL

If SELECT FINAL is too slow, consider these alternatives:

  • OPTIMIZE TABLE ... FINAL : This command forces a full merge of the table, applying all pending mutations. Run this periodically during off-peak hours to keep your data consistent without impacting query performance. This moves the overhead from query time to a scheduled maintenance window.
  • Materialized Views: Create materialized views that pre-aggregate and collapse your data. The view will store the results of the aggregation, so queries against the view will be much faster than using SELECT FINAL on the base table. The trade-off is that the view might not be perfectly up-to-date.
  • Accept eventual consistency: In many cases, you can simply accept that your data will be eventually consistent. The background merges will eventually catch up, and your queries will become more accurate over time. This is often the best approach for high-throughput, low-latency applications.

Deep Dive: Understanding the Internals

To truly grasp the impact of SELECT FINAL , it’s helpful to understand what happens under the hood. When you execute a SELECT query on a MergeTree table, ClickHouse reads data from multiple parts . Each part is a sorted collection of data. Without FINAL , ClickHouse simply reads the data from these parts and combines them, without considering whether any parts are marked for deletion or have been superseded by newer versions.

When you add FINAL to the query, ClickHouse performs the following steps:

  1. Identify all parts: It identifies all the data parts that are relevant to the query.
  2. Initiate a merge: It initiates a merge operation, combining all the selected parts into a single, consolidated part.
  3. Apply mutations: During the merge, it applies any pending mutations, such as deletions or updates.
  4. Return the result: Finally, it returns the result from the merged part.

This merge operation can be resource-intensive because it involves reading, processing, and writing large amounts of data. The more parts there are to merge, the longer it will take. That’s why it’s crucial to monitor your MergeTree tables and ensure that merges are happening regularly.

Real-World Use Cases

Let’s look at some real-world scenarios where SELECT FINAL can be a lifesaver:

  • Financial Transactions: In a financial system, you might use a CollapsingMergeTree to track account balances. When a transaction occurs, you insert a row with the transaction amount and a sign (+ for credit, - for debit). SELECT FINAL ensures that you always get the correct account balance, even if some transactions are still being processed.
  • Inventory Management: Imagine tracking inventory levels in a warehouse. You can use a CollapsingMergeTree to record stock additions and removals. SELECT FINAL provides an accurate snapshot of the current stock levels, preventing overselling or stockouts.
  • Clickstream Analysis: As shown in our example, tracking user activity on a website is a prime use case. SELECT FINAL helps you accurately determine the number of active users, the most popular pages, and other key metrics, even with a high volume of events.

Conclusion

SELECT FINAL is a powerful tool in the ClickHouse arsenal, especially when working with CollapsingMergeTree and other similar engines. It guarantees data consistency and accuracy, but it comes at a performance cost. Understanding when to use it – and when to avoid it – is crucial for building efficient and reliable data pipelines. So, experiment with it, measure its impact on your queries, and make informed decisions based on your specific needs. Happy querying!

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