Function array_unnest

src/backend/utils/adt/arrayfuncs.c:6135–6218

* UNNEST */

 * UNNEST
 */
Datum
array_unnest(PG_FUNCTION_ARGS)
{
	typedef struct
	{
		array_iter	iter;
		int			nextelem;
		int			numelems;
		int16		elmlen;
		bool		elmbyval;
		char		elmalign;
	} array_unnest_fctx;

	FuncCallContext *funcctx;
	array_unnest_fctx *fctx;
	MemoryContext oldcontext;

	/* stuff done only on the first call of the function */
	if (SRF_IS_FIRSTCALL())
	{
		AnyArrayType *arr;

		/* create a function context for cross-call persistence */
		funcctx = SRF_FIRSTCALL_INIT();

		/*
		 * switch to memory context appropriate for multiple function calls
		 */
		oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

		/*
		 * Get the array value and detoast if needed.  We can't do this
		 * earlier because if we have to detoast, we want the detoasted copy
		 * to be in multi_call_memory_ctx, so it will go away when we're done
		 * and not before.  (If no detoast happens, we assume the originally
		 * passed array will stick around till then.)
		 */
		arr = PG_GETARG_ANY_ARRAY_P(0);

		/* allocate memory for user context */
		fctx = (array_unnest_fctx *) palloc(sizeof(array_unnest_fctx));

		/* initialize state */
		array_iter_setup(&fctx->iter, arr);
		fctx->nextelem = 0;
		fctx->numelems = ArrayGetNItems(AARR_NDIM(arr), AARR_DIMS(arr));

		if (VARATT_IS_EXPANDED_HEADER(arr))
		{
			/* we can just grab the type data from expanded array */
			fctx->elmlen = arr->xpn.typlen;
			fctx->elmbyval = arr->xpn.typbyval;
			fctx->elmalign = arr->xpn.typalign;
		}
		else
			get_typlenbyvalalign(AARR_ELEMTYPE(arr),
								 &fctx->elmlen,
								 &fctx->elmbyval,