How to check the SQL database limit?

Checking SQL database limits can be done in several ways, depending on the specific DBMS (MySQL, PostgreSQL, SQL Server, etc.) and the limit of interest (size, number of connections, memory, etc.).

Common methods include:

• System views and tables: Most DBMSs provide system views or tables containing configuration and current state information.
• Configuration files: Limits are often set in the DBMS configuration files.
• Administration commands / Console utilities: Special commands or command-line utilities to obtain configuration information.

Examples for popular DBMSs:

MySQL:

SHOW VARIABLES LIKE 'max_connections'; // Connection limit
SHOW VARIABLES LIKE 'max_allowed_packet'; // Packet size limit
SHOW VARIABLES LIKE 'innodb_buffer_pool_size'; // InnoDB buffer pool size (related to memory)

PostgreSQL:

SHOW max_connections; // Connection limit
SHOW shared_buffers; // Shared buffers size (related to memory)
SHOW work_mem;       // Memory size for temporary tables and sorts

SQL Server:

-- Get configuration values
SELECT * FROM sys.configurations WHERE name LIKE '%connections%';
SELECT * FROM sys.configurations WHERE name LIKE '%memory%';

-- Max number of concurrent requests (related to connections)
SELECT @@MAX_PRECISION;

Database size limit:

Checking the actual size of the database:

• MySQL:

  SELECT table_schema "Database",
         ROUND(SUM(data_length + index_length) / 1024 / 1024, 2) "Size in MB"
  FROM information_schema.TABLES
  GROUP BY table_schema;
  

• PostgreSQL:

  SELECT pg_database.datname, pg_size_pretty(pg_database_size(pg_database.datname)) AS size
  FROM pg_database;
  

• SQL Server:

  SELECT
      DB_NAME(database_id) AS DatabaseName,
      CAST(SUM(size) * 8 / 1024.00 AS NUMERIC(18, 2)) AS SizeInMB
  FROM sys.master_files
  GROUP BY database_id;
  

It is important to know which specific limit is of interest, as there are many. For a complete understanding, refer to the official documentation of your specific DBMS version.