Distributed SQL over TiKV
TiKV is the storage layer for TiDB, a distributed HTAP SQL database. So far, we have only explained how a distributed transactional Key-Value database is implemented. However this is still far from serving a SQL database. We will explore and cover the following things in this chapter:
In this section we will see how the TiDB relational structure (i.e. SQL table records and indexes) are encoded into the Key-Value form in the latest version. We will also explore a new Key-Value format that is going to be implemented soon and some insights on even better Key-Value formats in future.
Distributed SQL (DistSQL)
Storing data in a distributed manner using TiKV only utilizes distributed I/O resources, while the TiDB node that receives SQL query is still in charge of processing all rows. We can go a step further by delegating some processing tasks into TiKV nodes. This way, we can utilize distributed CPU resources! In this section, we will take a look at these supported physical plan executors so far in TiKV and see how they enable TiDB executing SQL queries in a distributed way.
TiKV Query Execution Engine
When talking about executors we cannot ignore discussing the execution engine. Although executors running on TiKV are highly simplified and limited, we still need to carefully design the execution engine. It is critical to the performance of the system. This section will cover the traditional Volcano model execution engine used before TiKV 3.0, for example, how it works, pros and cons, and the architecture.
Vectorization is a technique that performs computing over a batch of values. By introducing vectorization into the execution engine, we will achieve higher performance. This section will introduce its theory and the architecture of the vectorized execution engine introduced in TiKV 3.0.