Function crosstab

contrib/tablefunc/tablefunc.c:353–600

 */
PG_FUNCTION_INFO_V1(crosstab);
Datum
crosstab(PG_FUNCTION_ARGS)
{
	char	   *sql = text_to_cstring(PG_GETARG_TEXT_PP(0));
	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
	Tuplestorestate *tupstore;
	TupleDesc	tupdesc;
	uint64		call_cntr;
	uint64		max_calls;
	AttInMetadata *attinmeta;
	SPITupleTable *spi_tuptable;
	TupleDesc	spi_tupdesc;
	bool		firstpass;
	char	   *lastrowid;
	int			i;
	int			num_categories;
	MemoryContext per_query_ctx;
	MemoryContext oldcontext;
	int			ret;
	uint64		proc;

	/* check to see if caller supports us returning a tuplestore */
	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
		ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("set-valued function called in context that cannot accept a set")));
	if (!(rsinfo->allowedModes & SFRM_Materialize))
		ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("materialize mode required, but it is not allowed in this context")));

	per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;

	/* Connect to SPI manager */
	if ((ret = SPI_connect()) < 0)
		/* internal error */
		elog(ERROR, "crosstab: SPI_connect returned %d", ret);

	/* Retrieve the desired rows */
	ret = SPI_execute(sql, true, 0);
	proc = SPI_processed;

	/* If no qualifying tuples, fall out early */
	if (ret != SPI_OK_SELECT || proc == 0)
	{
		SPI_finish();
		rsinfo->isDone = ExprEndResult;
		PG_RETURN_NULL();
	}

	spi_tuptable = SPI_tuptable;
	spi_tupdesc = spi_tuptable->tupdesc;

	/*----------
	 * The provided SQL query must always return three columns.
	 *
	 * 1. rowname
	 *	the label or identifier for each row in the final result