Function tablesample_init

src/backend/executor/nodeSamplescan.c:232–329

* Initialize the TABLESAMPLE method: evaluate params and call BeginSampleScan. */

 * Initialize the TABLESAMPLE method: evaluate params and call BeginSampleScan.
 */
static void
tablesample_init(SampleScanState *scanstate)
{
	TsmRoutine *tsm = scanstate->tsmroutine;
	ExprContext *econtext = scanstate->ss.ps.ps_ExprContext;
	Datum	   *params;
	Datum		datum;
	bool		isnull;
	uint32		seed;
	bool		allow_sync;
	int			i;
	ListCell   *arg;

	scanstate->donetuples = 0;
	params = (Datum *) palloc(list_length(scanstate->args) * sizeof(Datum));

	i = 0;
	foreach(arg, scanstate->args)
	{
		ExprState  *argstate = (ExprState *) lfirst(arg);

		params[i] = ExecEvalExprSwitchContext(argstate,
											  econtext,
											  &isnull);
		if (isnull)
			ereport(ERROR,
					(errcode(ERRCODE_INVALID_TABLESAMPLE_ARGUMENT),
					 errmsg("TABLESAMPLE parameter cannot be null")));
		i++;
	}

	if (scanstate->repeatable)
	{
		datum = ExecEvalExprSwitchContext(scanstate->repeatable,
										  econtext,
										  &isnull);
		if (isnull)
			ereport(ERROR,
					(errcode(ERRCODE_INVALID_TABLESAMPLE_REPEAT),
					 errmsg("TABLESAMPLE REPEATABLE parameter cannot be null")));

		/*
		 * The REPEATABLE parameter has been coerced to float8 by the parser.
		 * The reason for using float8 at the SQL level is that it will
		 * produce unsurprising results both for users used to databases that
		 * accept only integers in the REPEATABLE clause and for those who
		 * might expect that REPEATABLE works like setseed() (a float in the
		 * range from -1 to 1).
		 *
		 * We use hashfloat8() to convert the supplied value into a suitable
		 * seed.  For regression-testing purposes, that has the convenient
		 * property that REPEATABLE(0) gives a machine-independent result.
		 */
		seed = DatumGetUInt32(DirectFunctionCall1(hashfloat8, datum));
	}
	else
	{
		/* Use the seed selected by ExecInitSampleScan */