46.3. Memory Management
- SPI_palloc — allocate memory in the upper executor context
- SPI_repalloc — reallocate memory in the upper executor context
- SPI_pfree — free memory in the upper executor context
- SPI_copytuple — make a copy of a row in the upper executor context
- SPI_returntuple — prepare to return a tuple as a Datum
- SPI_modifytuple — create a row by replacing selected fields of a given row
- SPI_freetuple — free a row allocated in the upper executor context
— free a row set created by
SPI_executeor a similar function
- SPI_freeplan — free a previously saved prepared statement
PostgreSQL allocates memory within memory contexts, which provide a convenient method of managing allocations made in many different places that need to live for differing amounts of time. Destroying a context releases all the memory that was allocated in it. Thus, it is not necessary to keep track of individual objects to avoid memory leaks; instead only a relatively small number of contexts have to be managed.
palloc and related functions allocate memory from the “current” context.
SPI_connect creates a new memory context and makes it current.
SPI_finish restores the previous current memory context and destroys the context created by
SPI_connect. These actions ensure that transient memory allocations made inside your procedure are reclaimed at procedure exit, avoiding memory leakage.
However, if your procedure needs to return an object in allocated memory (such as a value of a pass-by-reference data type), you cannot allocate that memory using
palloc, at least not while you are connected to SPI. If you try, the object will be deallocated by
SPI_finish, and your procedure will not work reliably. To solve this problem, use
SPI_palloc to allocate memory for your return object.
SPI_palloc allocates memory in the “upper executor context”, that is, the memory context that was current when
SPI_connect was called, which is precisely the right context for a value returned from your procedure. Several of the other utility procedures described in this section also return objects created in the upper executor context.
SPI_connect is called, the private context of the procedure, which is created by
SPI_connect, is made the current context. All allocations made by
repalloc, or SPI utility functions (except as described in this section) are made in this context. When a procedure disconnects from the SPI manager (via
SPI_finish) the current context is restored to the upper executor context, and all allocations made in the procedure memory context are freed and cannot be used any more.