This page provides best practices for optimizing query performance in CockroachDB.
For guidance on deployment and data location techniques to minimize network latency, see Topology Patterns.
If you aren't sure whether SQL query performance needs to be improved on your cluster, see Identify slow queries.
DML best practices
Use multi-row statements instead of multiple single-row statements
DELETE statements, a single multi-row statement is faster than multiple single-row statements. Whenever possible, use multi-row statements for DML queries instead of multiple single-row statements.
For more information, see:
Bulk-insert best practices
INSERT statements for bulk-inserts into existing tables
To bulk-insert data into an existing table, batch multiple rows in one multi-row
INSERT statement. Experimentally determine the optimal batch size for your application by monitoring the performance for different batch sizes (10 rows, 100 rows, 1000 rows). Do not include bulk
INSERT statements within an explicit transaction.
You can also use the
IMPORT INTO statement to bulk-insert CSV data into an existing table.
For more information, see Insert Multiple Rows.
INSERT queries can lead to long-running transactions that result in transaction retry errors. If a multi-row
INSERT query results in an error code
40001 with the message
"transaction deadline exceeded", we recommend breaking up the query up into smaller batches of rows.
IMPORT instead of
INSERT for bulk-inserts into new tables
To bulk-insert data into a brand new table, the
IMPORT statement performs better than
Bulk-update best practices
Use batch updates to delete a large number of rows
To delete a large number of rows, we recommend iteratively deleting batches of rows until all of the unwanted rows are deleted. For an example, see Bulk-update Data.
Bulk-delete best practices
TRUNCATE instead of
DELETE to delete all rows in a table
TRUNCATE statement removes all rows from a table by dropping the table and recreating a new table with the same name. This performs better than using
DELETE, which performs multiple transactions to delete all rows.
Use batch deletes to delete a large number of rows
To delete a large number of rows, we recommend iteratively deleting batches of rows until all of the unwanted rows are deleted. For an example, see Bulk-delete Data.
Batch delete "expired" data
CockroachDB does not support Time to Live (TTL) on table rows. To delete "expired" rows, we recommend automating a batch delete process with a job scheduler like
cron. For an example, see Batch-delete "expired" data.
Assign column families
A column family is a group of columns in a table that is stored as a single key-value pair in the underlying key-value store.
When a table is created, all columns are stored as a single column family. This default approach ensures efficient key-value storage and performance in most cases. However, when frequently updated columns are grouped with seldom updated columns, the seldom updated columns are nonetheless rewritten on every update. Especially when the seldom updated columns are large, it's therefore more performant to assign them to a distinct column family.
Unique ID best practices
The best practices for generating unique IDs in a distributed database like CockroachDB are very different than for a legacy single-node database. Traditional approaches for generating unique IDs for legacy single-node databases include:
- Using the
SERIALpseudo-type for a column to generate random unique IDs. This can result in a performance bottleneck because IDs generated temporally near each other have similar values and are located physically near each other in a table's storage.
- Generating monotonically increasing
INTIDs by using transactions with roundtrip
INSERT INTO tbl (id, …) VALUES ((SELECT max(id)+1 FROM tbl), …). This has a very high performance cost since it makes all
INSERTtransactions wait for their turn to insert the next ID. You should only do this if your application really does require strict ID ordering. In some cases, using Change Data Capture (CDC) can help avoid the requirement for strict ID ordering. If you can avoid the requirement for strict ID ordering, you can use one of the higher performance ID strategies outlined below.
The approaches described above are likely to create hotspots for both reads and writes in CockroachDB. To avoid this issue, we recommend the following approaches (listed in order from best to worst performance).
|1. Use multi-column primary keys||Potentially fastest, if done right||Complex, requires up-front design and testing to ensure performance|
||Good performance; spreads load well; easy choice||May leave some performance on the table; requires other columns to be useful in queries|
||Easy to query against; familiar design||Slower performance than the other options; higher chance of transaction contention|
Use multi-column primary keys
A well-designed multi-column primary key can yield even better performance than a UUID primary key, but it requires more up-front schema design work. To get the best performance, ensure that any monotonically increasing field is located after the first column of the primary key. When done right, such a composite primary key should result in:
- Enough randomness in your primary key to spread the table data / query load relatively evenly across the cluster, which will avoid hotspots. By "enough randomness" we mean that the prefix of the primary key should be relatively uniformly distributed over its domain. Its domain should have at least as many elements as you have nodes.
- A monotonically increasing column that is part of the primary key (and thus indexed) which is also useful in your queries.
For example, consider a social media website. Social media posts are written by users, and on login the user's last 10 posts are displayed. A good choice for a primary key might be
(username, post_timestamp). For example:
> CREATE TABLE posts ( username STRING, post_timestamp TIMESTAMP, post_id INT, post_content STRING, CONSTRAINT posts_pk PRIMARY KEY(username, post_timestamp) );
This would make the following query efficient.
> SELECT * FROM posts WHERE username = 'alyssa' ORDER BY post_timestamp DESC LIMIT 10;
username | post_timestamp | post_id | post_content +----------+---------------------------+---------+--------------+ alyssa | 2019-07-31 18:01:00+00:00 | ... | ... alyssa | 2019-07-30 10:22:00+00:00 | ... | ... alyssa | 2019-07-30 09:12:00+00:00 | ... | ... alyssa | 2019-07-29 13:48:00+00:00 | ... | ... alyssa | 2019-07-29 13:47:00+00:00 | ... | ... alyssa | 2019-07-29 13:46:00+00:00 | ... | ... alyssa | 2019-07-29 13:43:00+00:00 | ... | ... ... Time: 924µs
To see why, let's look at the
EXPLAIN output. It shows that the query is fast because it does a point lookup on the indexed column
username (as shown by the line
spans | /"alyssa"-...). Furthermore, the column
post_timestamp is already in an index, and sorted (since it's a monotonically increasing part of the primary key).
> EXPLAIN (VERBOSE) SELECT * FROM posts WHERE username = 'alyssa' ORDER BY post_timestamp DESC LIMIT 10;
info ---------------------------------------------------------------- distribution: local vectorized: true • revscan columns: (username, post_timestamp, post_id, post_content) ordering: -post_timestamp estimated row count: 10 (missing stats) table: posts@posts_pk spans: /"alyssa"-/"alyssa"/PrefixEnd limit: 10 (10 rows) Time: 1ms total (execution 1ms / network 0ms)
Note that the above query also follows the indexing best practice of indexing all columns in the
UUID to generate unique IDs
> CREATE TABLE t1 (id UUID PRIMARY KEY DEFAULT gen_random_uuid(), name STRING);
> INSERT INTO t1 (name) VALUES ('a'), ('b'), ('c');
> SELECT * FROM t1;
+--------------------------------------+------+ | id | name | +--------------------------------------+------+ | 60853a85-681d-4620-9677-946bbfdc8fbc | c | | 77c9bc2e-76a5-4ebc-80c3-7ad3159466a1 | b | | bd3a56e1-c75e-476c-b221-0da9d74d66eb | a | +--------------------------------------+------+ (3 rows)
INSERT with the
RETURNING clause to generate unique IDs
If something prevents you from using multi-column primary keys or
UUIDs to generate unique IDs, you might resort to using
SELECTs to return IDs. Instead, use the
RETURNING clause with the
INSERT statement as shown below for improved performance.
Generate monotonically-increasing unique IDs
Suppose the table schema is as follows:
> CREATE TABLE X ( ID1 INT, ID2 INT, ID3 INT DEFAULT 1, PRIMARY KEY (ID1,ID2) );
The common approach would be to use a transaction with an
INSERT followed by a
> BEGIN; > INSERT INTO X VALUES (1,1,1) ON CONFLICT (ID1,ID2) DO UPDATE SET ID3=X.ID3+1; > SELECT * FROM X WHERE ID1=1 AND ID2=1; > COMMIT;
However, the performance best practice is to use a
RETURNING clause with
INSERT instead of the transaction:
> INSERT INTO X VALUES (1,1,1),(2,2,2),(3,3,3) ON CONFLICT (ID1,ID2) DO UPDATE SET ID3=X.ID3 + 1 RETURNING ID1,ID2,ID3;
Generate random unique IDs
Suppose the table schema is as follows:
> CREATE TABLE X ( ID1 INT, ID2 INT, ID3 INT DEFAULT unique_rowid(), PRIMARY KEY (ID1,ID2) );
The common approach to generate random Unique IDs is a transaction using a
> BEGIN; > INSERT INTO X VALUES (1,1); > SELECT * FROM X WHERE ID1=1 AND ID2=1; > COMMIT;
RETURNING clause with
INSERT instead of the transaction:
> INSERT INTO X VALUES (1,1),(2,2),(3,3) RETURNING ID1,ID2,ID3;
Secondary index best practices
Join best practices
Subquery best practices
Table scans best practices
SELECT * for large tables
For large tables, avoid table scans (that is, reading the entire table data) whenever possible. Instead, define the required fields in a
Suppose the table schema is as follows:
> CREATE TABLE accounts ( id INT, customer STRING, address STRING, balance INT nominee STRING );
Now if we want to find the account balances of all customers, an inefficient table scan would be:
> SELECT * FROM ACCOUNTS;
This query retrieves all data stored in the table. A more efficient query would be:
> SELECT CUSTOMER, BALANCE FROM ACCOUNTS;
This query returns the account balances of the customers.
SELECT DISTINCT for large tables
SELECT DISTINCT allows you to obtain unique entries from a query by removing duplicate entries. However,
SELECT DISTINCT is computationally expensive. As a performance best practice, use
SELECT with the
WHERE clause instead.
AS OF SYSTEM TIME to decrease conflicts with long-running queries
If you have long-running queries (such as analytics queries that perform full table scans) that can tolerate slightly out-of-date reads, consider using the
... AS OF SYSTEM TIME clause. Using this, your query returns data as it appeared at a distinct point in the past and will not cause conflicts with other concurrent transactions, which can increase your application's performance.
AS OF SYSTEM TIME returns historical data, your reads might be stale.
Understanding and avoiding transaction contention
Transaction contention occurs when the following three conditions are met:
- There are multiple concurrent transactions or statements (sent by multiple clients connected simultaneously to a single CockroachDB cluster).
- They operate on the same data, specifically over table rows with the same index key values (either on primary keys or secondary indexes) or using index key values that are close to each other, and thus place the indexed data on the same data ranges.
- At least some of the transactions write or modify the data.
A set of transactions that all contend on the same keys will be limited in performance to the maximum processing speed of a single node (limited horizontal scalability). Non-contended transactions are not affected in this way.
There are two levels of contention:
Transactions that operate on the same range but different index key values will be limited by the overall hardware capacity of a single node (the range lease holder).
Transactions that operate on the same index key values (specifically, that operate on the same column family for a given index key) will be more strictly serialized to obey transaction isolation semantics.
Transaction contention can also increase the rate of transaction restarts, and thus make the proper implementation of client-side transaction retries more critical.
To avoid contention, multiple strategies can be applied:
Use index key values with a more random distribution of values, so that transactions over different rows are more likely to operate on separate data ranges. See the SQL FAQs on row IDs for suggestions.
Make transactions smaller, so that each transaction has less work to do. In particular, avoid multiple client-server exchanges per transaction. For example, use common table expressions to group multiple
UPSERTclauses together in a single SQL statement.
- For an example showing how to break up large transactions in an application, see Break up large transactions into smaller units of work.
- If you are experiencing contention (retries) when doing bulk deletes, see Bulk-delete data.
In combination with the above, if you are able to send all of the statements in your transaction in a single batch, CockroachDB can automatically retry the transaction for you.
SELECT FOR UPDATEstatement in scenarios where a transaction performs a read and then updates the row(s) it just read. It orders transactions by controlling concurrent access to one or more rows of a table. It works by locking the rows returned by a selection query, such that other transactions trying to access those rows are forced to wait for the transaction that locked the rows to finish. These other transactions are effectively put into a queue that is ordered based on when they try to read the value of the locked row(s).
When replacing values in a row, use
UPSERTand specify values for all columns in the inserted rows. This will usually have the best performance under contention, compared to combinations of
Increase normalization of the data to place parts of the same records that are modified by different transactions in different tables. Note however that this is a double-edged sword, because denormalization can also increase performance by creating multiple copies of often-referenced data in separate ranges.
If the application strictly requires operating on very few different index key values, consider using
ALTER ... SPLIT ATso that each index key value can be served by a separate group of nodes in the cluster.
It is always best to avoid contention as much as possible via the design of the schema and application. However, sometimes contention is unavoidable. To maximize performance in the presence of contention, you'll need to maximize the performance of a single range. To achieve this, multiple strategies can be applied:
- Minimize the network distance between the replicas of a range, possibly using zone configs and partitioning.
- Use the fastest storage devices available.
- If the contending transactions operate on different keys within the same range, add more CPU power (more cores) per node. Note however that this is less likely to provide an improvement if the transactions all operate on the same key.