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TiDB Best Practices |
Learn the best practices of using TiDB. |
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This document summarizes the best practices of using TiDB, including the use of SQL and optimization tips for Online Analytical Processing (OLAP) and Online Transactional Processing (OLTP) scenarios, especially the optimization options specific for TiDB.
Before you read this document, it is recommended that you read three blog posts that introduce the technical principles of TiDB:
Database is a generic infrastructure system. It is important to consider various user scenarios during the development process and to modify the data parameters or the way to use according to actual situations in specific business scenarios.
TiDB is a distributed database compatible with the MySQL protocol and syntax. But with the internal implementation and supporting of distributed storage and transactions, the way of using TiDB is different from MySQL.
The best practices are closely related to its implementation principles. It is recommended that you learn some of the basic mechanisms, including the Raft consensus algorithm, distributed transactions, data sharding, load balancing, the mapping solution from SQL to Key-Value (KV), the implementation method of secondary indexing, and distributed execution engines.
This section is an introduction to these concepts. For detailed information, refer to PingCAP blog posts.
Raft is a consensus algorithm that ensures data replication with strong consistency. At the bottom layer, TiDB uses Raft to replicate data. TiDB writes data to the majority of the replicas before returning the result of success. In this way, even though a few replicas might get lost, the system still has the latest data. For example, if there are three replicas, the system does not return the result of success until data has been written to two replicas. Whenever a replica is lost, at least one of the remaining two replicas have the latest data.
To store three replicas, compared with the replication of Source-Replica, Raft is more efficient. The write latency of Raft depends on the two fastest replicas, instead of the slowest one. Therefore, the implementation of geo-distributed and multiple active data centers becomes possible by using the Raft replication. In the typical scenario of three data centers distributing in two sites, to guarantee the data consistency, TiDB just needs to successfully write data into the local data center and the closer one, instead of writing to all three data centers. However, this does not mean that cross-data center deployment can be implemented in any scenario. When the amount of data to be written is large, the bandwidth and latency between data centers become the key factors. If the write speed exceeds the bandwidth or the latency is too high, the Raft replication mechanism still cannot work well.
TiDB provides complete distributed transactions and the model has some optimizations on the basis of Google Percolator. This document introduces the following features:
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Optimistic transaction model
TiDB's optimistic transaction model does not detect conflicts until the commit phase. If there are conflicts, the transaction needs retry. But this model is inefficient if the conflict is severe, because operations before retry are invalid and need to repeat.
Assume that the database is used as a counter. High access concurrency might lead to severe conflicts, resulting in multiple retries or even timeouts. Therefore, in the scenario of severe conflicts, it is recommended to use the pessimistic transaction mode or to solve problems at the system architecture level, such as placing counter in Redis. Nonetheless, the optimistic transaction model is efficient if the access conflict is not very severe.
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Pessimistic transaction model
In TiDB, the pessimistic transaction model has almost the same behavior as in MySQL. The transaction applies a lock during the execution phase, which avoids retries in conflict situations and ensures a higher success rate. By applying the pessimistic locking, you can also lock data in advance using
SELECT FOR UPDATE.However, if the application scenario has fewer conflicts, the optimistic transaction model has better performance.
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Transaction size limit
As distributed transactions need to conduct two-phase commit and the bottom layer performs Raft replication, if a transaction is very large, the commit process would be quite slow, and the following Raft replication process is thus stuck. To avoid this problem, the transaction size is limited:
- A transaction is limited to 5,000 SQL statements (by default)
- Each Key-Value entry is no more than 6 MB (by default)
- The total size of Key-Value entries is no more than 10 GB.
You can find similar limits in Google Cloud Spanner.
TiKV automatically shards bottom-layered data according to the range of keys. Each Region is a range of keys, which is a left-closed and right-open interval, [StartKey, EndKey). When the amount of Key-Value pairs in a Region exceeds a certain value, the Region automatically splits into two.
Placement Driver (PD) balances the load of the cluster according to the status of the entire TiKV cluster. The unit of scheduling is Region and the logic is the strategy configured by PD.
TiDB automatically maps the SQL structure into Key-Value structure. For details, see TiDB Internal (II) - Computing.
Simply put, TiDB performs the following operations:
- A row of data is mapped to a Key-Value pair. The key is prefixed with
TableIDand suffixed with the row ID. - An index is mapped as a Key-Value pair. The key is prefixed with
TableID+IndexIDand suffixed with the index value.
The data or indexes in the same table have the same prefix. These Key-Values are at adjacent positions in the key space of TiKV. Therefore, when the amount of data to be written is large and all is written to one table, the write hotspot is created. The situation gets worse when some index values of the continuous written data is also continuous (for example, fields that increase with time, like update time), which creates a few write hotspots and becomes the bottleneck of the entire system.
Similarly, if all data is read from a focused small range (for example, the continuous tens or hundreds of thousands of rows of data), an access hotspot of data is likely to occur.
TiDB supports the complete secondary indexes, which are also global indexes. Many queries can be optimized by index. Thus, it is important for applications to make good use of secondary indexes.
Lots of MySQL experience is also applicable to TiDB. It is noted that TiDB has its unique features. The following are a few notes when using secondary indexes in TiDB.
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The more secondary indexes, the better?
Secondary indexes can speed up queries, but adding an index has side effects. The previous section introduces the storage model of indexes. For each additional index, there will be one more Key-Value when inserting a piece of data. Therefore, the more indexes, the slower the writing speed and the more space it takes up.
In addition, too many indexes affects the runtime of the optimizer, and inappropriate indexes mislead the optimizer. Thus, more secondary indexes does not mean better performance.
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Which columns should create indexes?
As is mentioned above, index is important but the number of indexes should be proper. You must create appropriate indexes according to the application characteristics. In principle, you need to create an index on the columns involved in the query to improve the performance. The following are situations that need to create indexes:
- For columns with a high degree of differentiation, filtered rows are remarkably reduced through indexes.
- If there are multiple query criteria, you can choose composite indexes. Note to put the columns with the equivalent condition before composite indexes.
For example, if a commonly used query is
select * from t where c1 = 10 and c2 = 100 and c3 > 10, you can create a composite indexIndex cidx (c1, c2, c3). In this way, you can use the query condition to create an index prefix and then scan. -
The difference between querying through indexes and directly scanning the table
TiDB has implemented global indexes, so indexes and data of the table are not necessarily on the same data sharding. When querying through indexes, it should firstly scan indexes to get the corresponding row ID and then use the row ID to get the data. Thus, this method involves two network requests and has a certain performance overhead.
If the query involves lots of rows, scanning index proceeds concurrently. When the first batch of results is returned, getting the data of the table can then proceed. Therefore, this is a parallel + pipeline model. Though the two accesses create overhead, the latency is not high.
The following two conditions do not have the problem of two accesses:
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Columns of the index have already met the query requirement. Assume that the
ccolumn on thettable has an index and the query isselect c from t where c > 10;. At this time, all needed data can be obtained if you access the index. This situation is calledCovering Index. But if you focus more on the query performance, you can put into index a portion of columns that do not need to be filtered but need to be returned in the query result, creating composite index. Takeselect c1, c2 from t where c1 > 10;as an example. You can optimize this query by creating composite indexIndex c12 (c1, c2). -
The primary key of the table is integer. In this case, TiDB uses the value of the primary key as row ID. Thus, if the query condition is on the primary key, you can directly construct the range of the row ID, scan the table data, and get the result.
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Query concurrency
As data is distributed across many Regions, queries run in TiDB concurrently. But the concurrency by default is not high in case it consumes lots of system resources. Besides, the OLTP query usually does not involve a large amount of data and the low concurrency is enough. But for the OLAP query, the concurrency is high and TiDB modifies the query concurrency through the following system variables:
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tidb_distsql_scan_concurrency:The concurrency of scanning data, including scanning the table and index data.
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If it needs to access the index to get row IDs before accessing the table data, it uses a batch of row IDs as a single request to access the table data. This parameter sets the size of a batch. The larger batch increases latency, while the smaller one might lead to more queries. The proper size of this parameter is related to the amount of data that the query involves. Generally, no modification is required.
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tidb_index_lookup_concurrency:If it needs to access the index to get row IDs before accessing the table data, the concurrency of getting data through row IDs every time is modified through this parameter.
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Ensure the order of results through indexes
You can use indexes to filter or sort data. Firstly, get row IDs according to the index order. Then, return the row content according to the return order of row IDs. In this way, the returned results are ordered according to the index column. It has been mentioned earlier that the model of scanning index and getting row is parallel + pipeline. If the row is returned according to the index order, a high concurrency between two queries does not reduce latency. Thus, the concurrency is low by default, but it can be modified through the
tidb_index_serial_scan_concurrencyvariable. -
Reverse index scan
TiDB supports scanning an ascending index in reverse order, at a speed slower than normal scan by 20%. If the data is changed frequently and thus too many versions exist, the performance overhead might be higher. It is recommended to avoid reverse index scans as much as possible.
In the last section, we discussed some basic implementation mechanisms of TiDB and their influence on usage. This section introduces specific usage scenarios and operation practices, from deployment to application usage.
Before deployment, read Software and Hardware Requirements.
It is recommended to deploy the TiDB cluster using TiUP. This tool can deploy, stop, destroy, and upgrade the whole cluster, which is quite convenient. It is not recommended to manually deploy the TiDB cluster, which might be troublesome to maintain and upgrade later.
To improve the write performance during the import process, you can tune TiKV's parameters as stated in Tune TiKV Memory Parameter Performance.
As mentioned before, TiDB limits the size of a single transaction in the Key-Value layer. As for the SQL layer, a row of data is mapped to a Key-Value entry. For each additional index, one more Key-Value entry is added.
Note:
When you set the size limit for transactions, you need to consider the overhead of TiDB encoding and the extra transaction key. It is recommended that the number of rows of each transaction is less than 200 and the data size of a single row is less than 100 KB; otherwise, the performance is bad.
It is recommended to split statements into batches or add a limit to the statements, whether they are INSERT, UPDATE or DELETE statements.
When deleting a large amount of data, it is recommended to use Delete from t where xx limit 5000;. It deletes through the loop and use Affected Rows == 0 as a condition to end the loop.
If the amount of data that needs to be deleted at a time is large, this loop method gets slower and slower because each deletion traverses backward. After deleting the previous data, lots of deleted flags remain for a short period (then all is cleared by Garbage Collection) and affect the following DELETE statement. If possible, it is recommended to refine the WHERE condition. Assume that you need to delete all data on 2017-05-26, you can use the following statements:
for i from 0 to 23:
while affected_rows > 0:
delete from t where insert_time >= i:00:00 and insert_time < (i+1):00:00 limit 5000;
affected_rows = select affected_rows()This pseudocode means to split huge chunks of data into small ones and then delete, so that the earlier Delete statements do not affect the later ones.
For query requirements and specific statements, refer to System Variables.
You can control the concurrency of SQL execution through the SET statement and the selection of the Join operator through hints.
In addition, you can also use MySQL's standard index selection, the hint syntax, or control the optimizer to select indexes through Use Index/Ignore Index hint.
If the application scenario has both OLTP and OLAP workloads, you can send the OLTP request and OLAP request to different TiDB servers, diminishing the impact of OLAP on OLTP. It is recommended to use machines with high-performance hardware (for example, more processor cores and larger memory) for the TiDB server that processes OLAP workloads.
To completely isolate OLTP and OLAP workloads, it is recommended to run OLAP applications on TiFlash. TiFlash is a columnar storage engine with great performance on OLAP workloads. TiFlash can achieve physical isolation on the storage layer and guarantees consistent reads.
The monitoring metrics is the best method to learn the status of the system. It is recommended that you deploy the monitoring system along with your TiDB cluster.
TiDB uses Grafana + Prometheus to monitor the system status. The monitoring system is automatically deployed and configured if you deploy TiDB using TiUP.
There are lots of items in the monitoring system, the majority of which are for TiDB developers. You do not have to understand these items without an in-depth knowledge of the source code. Some items that are related to applications or to the state of system key components are selected and put in a separate overview panel for users.
In addition to monitoring, you can also view the system logs. The three components of TiDB, tidb-server, tikv-server, and pd-server, each has a --log-file parameter. If this parameter has been configured when the cluster is started, logs are stored in the file configured by the parameter and log files are automatically archived on a daily basis. If the --log-file parameter has not been configured, the log is output to stderr.
Starting from TiDB 4.0, TiDB provides TiDB Dashboard UI to improve usability. You can access TiDB Dashboard by visiting http://${PD_IP}:${PD_PORT}/dashboard in your browser. TiDB Dashboard provides features such as viewing cluster status, performance analysis, traffic visualization, cluster diagnostics, and log searching.
The best way to learn about a system or solve the problem is to read its documentation and understand its implementation principles.
TiDB has a large number of official documents both in Chinese and English. If you have met an issue, you can start from FAQ and TiDB Cluster Troubleshooting Guide. You can also search the issue list or create an issue in TiDB repository on GitHub.
TiDB also has many useful ecosystem tools. See Ecosystem Tool Overview for details.
For more articles on the technical details of TiDB, see the PingCAP official blog site.
TiDB is suitable for the following scenarios:
- The data volume is too large for a standalone database
- You do not want to do sharding
- The access mode has no obvious hotspot
- Transactions, strong consistency, and disaster recovery are required
- You hope to have real-time Hybrid Transaction/Analytical Processing (HTAP) analytics and reduce storage links