SPI_executeor a similar function
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.
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
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.