As your application scales, you may need to use sharding to distribute load and maintain stable performance under heavy workloads. This topic describes logical tables, their use cases, and how to configure them.
Usage notes
- You must perform operations on a logical table within its corresponding logical database.Note A logical table can be used like a standard, single table.
- You can use the SQL Console to query, export, and modify data in logical tables.
- To request permissions for a data source, you can apply for permissions on the logical database directly. This grants access to all its associated physical databases.
Use cases
- Table sharding within a single physical database.
- Database and table sharding. This is the most common use case.Note
- Physical tables are distributed evenly across physical databases, with table names typically incrementing sequentially.
- Each database shard contains tables with the same naming pattern. For example, each database might contain 12 sharded tables, numbered 1 through 12.
- Database sharding without table sharding. In this scenario, each database shard contains a single physical table with the same name.
Definition
When a logical table contains multiple physical tables, they are distributed evenly across the physical databases based on the following rules:
- The number of tables is typically a power of two.
- Table names typically use a suffix with an underscore followed by a four-digit number, such as
_xxxx. The number is zero-padded and increments from 0. - The total number of tables must be evenly divisible by the number of databases. For example, if you have 1,024 tables and 32 databases, each database holds 32 physical tables. However, if you have 1,024 tables and 33 databases, the tables cannot be distributed evenly, and the logical table cannot be created.
Configuration
- Initialization After configuring a logical database, the system automatically extracts logical tables based on the schema of the physical tables. The extraction follows these rules:
- Each physical table can be included in only one logical table configuration.
- All physical tables within a logical table must have an identical schema, including column names and column types. If the schemas mismatch, the tables cannot be aggregated, and the system sends an alert.
Note You cannot modify a logical database's configuration after creation. - Add a logical table
If a logical table is missing after initialization, you can manually trigger an incremental extraction. Search for the logical database and find the target database in the results list. In the Actions column, click .
FAQ
- Q: An existing logical table is not appearing in the table list. How can I find it?
- A: In the top navigation bar of the Data Management (DMS) console, click the search icon to open the Global Search page. Find the target logical database, and in the Actions column, click .