47.3. 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
SPI_freetuptable — free a row set created by SPI_execute or 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 C function are reclaimed at C function exit, avoiding memory leakage.

However, if your C function 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 C function 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 C function. Several of the other utility functions described in this section also return objects created in the upper executor context.

When SPI_connect is called, the private context of the C function, which is created by SPI_connect, is made the current context. All allocations made by palloc, repalloc, or SPI utility functions (except as described in this section) are made in this context. When a C function disconnects from the SPI manager (via SPI_finish) the current context is restored to the upper executor context, and all allocations made in the C function memory context are freed and cannot be used any more.