How To Limit The Number Of Rows Returned In BigQuery

BigQuery is one of the most powerful cloud-based data warehousing and analytics platforms, but with great power comes great responsibility. One of the most common challenges when working with BigQuery is managing large data sets. This can lead to slow query performance, increased cost, and potential resource exhaustion.

Fortunately, BigQuery provides a simple way to limit the number of rows returned by a query: the LIMIT clause. In this post, we’ll look at how to use the LIMIT clause in BigQuery to optimize query performance and control the amount of data returned.

What is the LIMIT Clause in BigQuery?

The LIMIT clause in BigQuery is used to limit the number of rows returned by a query. It is placed at the end of the query, after the FROM clause, and before the ORDER BY clause (if present). The LIMIT clause is specified using the following syntax:

SELECT <column_list>
FROM <table_name>
LIMIT <integer>

The <integer> value in the LIMIT clause is the maximum number of rows that will be returned by the query. If the query would return more than <integer> rows, only the first <integer> rows are returned.

When to Use LIMIT in BigQuery

The LIMIT clause is useful in several situations, including:

• When you want to test a query before running it on the entire data set
• When you want to limit the amount of data returned to improve query performance
• When you want to limit the cost of a query by controlling the amount of data processed

In each of these situations, the LIMIT clause is a simple and effective way to limit the number of rows returned by a query.

Example: Using the LIMIT Clause in BigQuery

Let’s take a look at an example of how to use the LIMIT clause in BigQuery. Suppose you have a table mytable that contains 100 million rows, and you want to retrieve the first 10 rows. You can do this using the following query:

SELECT *
FROM mytable
LIMIT 10

This query will return the first 10 rows from the mytable table. Note that the order of the rows returned by the query is determined by the order in which the data is stored in BigQuery. If you want to order the results, you can use the ORDER BY clause, as shown in the following example:

SELECT *
FROM mytable
ORDER BY column1
LIMIT 10

This query will return the first 10 rows from the mytable table, ordered by the values in column1.

Using LIMIT with OFFSET

In addition to the LIMIT clause, BigQuery also provides the OFFSET clause, which allows you to specify the starting row of the result set. This can be useful when you want to retrieve a subset of rows from a large data set, but not necessarily the first few rows.

The OFFSET clause is specified after the LIMIT clause, and uses the following syntax:

SELECT <column_list>
FROM <table_name>
LIMIT <integer>
OFFSET <integer>

The first in the LIMIT clause specifies the maximum number of rows to return, and the second in the OFFSET clause specifies the starting row of the result set.

For example, suppose you want to retrieve the rows from mytable starting at row 11 and ending at row 20. You can use the following query:

SELECT *
FROM mytable
LIMIT 10
OFFSET 10

This query will skip the first 10 rows and return the next 10 rows from mytable. Note that the OFFSET value must be less than the total number of rows in the table, otherwise an error will be returned.

Paging through data using LIMIT and OFFSET

Paging is a common technique used in web applications to display data in smaller, more manageable chunks instead of showing all the data at once. In BigQuery, paging can be achieved using the LIMIT and OFFSET clauses. By using these clauses together, you can retrieve a subset of rows from a table and display them on a page, and then use the OFFSET value to retrieve the next subset of rows for the next page.

This process can be repeated until all the rows in the table have been displayed. Paging with LIMIT and OFFSET is especially useful when working with large data sets, as it allows you to break up the data into smaller, more manageable pieces and display them in a user-friendly manner. However, it’s important to keep in mind that as the OFFSET value increases, the query performance may decrease, so it’s best to use paging only when necessary and with appropriate tuning.

Here’s a quick example of implementing paging in this way

SELECT * FROM mytable LIMIT 10 OFFSET (10 * (current_page - 1));

Conclusion

The LIMIT clause in BigQuery is a simple and effective way to limit the number of rows returned by a query. It can be used to optimize query performance, control the amount of data returned, and limit the cost of a query by controlling the amount of data processed.

Whether you are working with small or large data sets, the LIMIT clause is an important tool to have in your BigQuery toolkit. Try using it in your next BigQuery project and see how it can help improve your query performance and reduce costs.