memorycontext

Memory context is a hierarchical memory allocation framework used by certain software systems to group allocations that share a common lifetime. A memory context represents a pool or arena from which memory blocks are allocated. Contexts can be nested; child contexts are allocated from a parent and can be reset or freed independently, allowing bulk deallocation of all memory allocated within a subtree.

In PostgreSQL, memory contexts play a central role in managing memory for queries and backend processes. The system uses a current memory context for the thread, and code creates subcontexts for specific tasks such as parsing, planning, or execution. Allocations are performed through a facility that attaches the result to the current context. When a phase completes, the corresponding context is reset or deleted, automatically freeing all memory allocated within that context without individually freeing each block.

Implementation typically uses a linked list of blocks and bookkeeping fields to track usage, block size, and a parent pointer. A MemoryContext may provide specialized allocators for small allocations or for frequent allocations. The design supports deterministic cleanup, avoids cross-context leaks, and reduces the need for frequent malloc/free pairs, improving performance in long-running processes.

Limitations include the need for careful design to avoid dangling pointers in long-lived contexts, and potential complexity when objects cross context boundaries. The concept is common in databases and server runtimes and is also seen in other languages as arena- or region-based allocators.

See also: memory arena, region-based allocator, allocator, PostgreSQL MemoryContext API.