When is memory allocated in PostgreSQL?

In PostgreSQL, the system allocates memory at various stages during query execution and other database operations. Below are key points where PostgreSQL allocates memory:

1. Connection Establishment:

   • When a client establishes a connection to the PostgreSQL server, the system allocates memory to manage the connection and related resources. 
   • This includes memory for managing session state, query processing, and connection-specific data structures.

2. Parsing and Planning:

   • When PostgreSQL parses and analyzes a SQL statement, it allocates memory to build the internal representation of the query. This structure is known as the parse tree or query tree.
   • Additionally, PostgreSQL allocates memory during the planning phase to generate an execution plan for the query.

3. Execution Plan:

   • After planning the query, PostgreSQL allocates memory to store the execution plan. This plan includes various data structures and buffers. 
   • PostgreSQL also allocates memory for storing join plans, sort plans, index scans, and other components necessary to execute the query.

4. Query Execution:

   • During execution, PostgreSQL allocates memory for intermediate result sets, temporary tables, and sorting operations and other processing tasks..
   • It may also allocate memory to cache frequently accessed data or maintain internal structures used for query optimization.

5. Result Sets:

   • When PostgreSQL generates query results, it allocates memory to hold the result set.
   • The size of the result set depends on the number of rows returned and the size of the data being returned.

6. Background Processes:

   • PostgreSQL runs various background processes such as autovacuum. These processes handle maintenance tasks and manage system resources.
   • The background processes require memory and may allocate it as needed during their operation.

7. Buffer Cache:

   • PostgreSQL maintains a buffer cache to store frequently accessed data pages from disk.
   • It allocates memory for these buffered data pages, which helps reduce disk I/O and improve performance.

It’s important to note that memory allocation in PostgreSQL is dynamic and depends on the specific workload, query complexity, system configuration, and available resources. PostgreSQL manages memory efficiently by utilizing memory pools and releasing memory when it is no longer needed. 
Configuration parameters such as shared_buffers, work_mem, and maintenance_work_mem play a critical role in determining memory usage and allocation strategy.
Monitoring memory usage and tuning these parameters helps ensure efficient memory utilization. This also helps prevent issues like excessive memory consumption or out-of-memory errors.