Function table_scan_sample_next_block

src/include/access/tableam.h:2287–2299

* Prepare to fetch tuples from the next block in a sample scan. Returns false * if the sample scan is finished, true otherwise. `scan` needs to have been * started via table_beginscan_sampling(). * * This will call the TsmRoutine's NextSampleBlock() callback if necessary * (i.e. NextSampleBlock is not NULL), or perform a sequential scan over the * underlying relation. */

 * underlying relation.
 */
static inline bool
table_scan_sample_next_block(TableScanDesc scan,
							 struct SampleScanState *scanstate)
{
	/*
	 * We don't expect direct calls to table_scan_sample_next_block with valid
	 * CheckXidAlive for catalog or regular tables.  See detailed comments in
	 * xact.c where these variables are declared.
	 */
	if (unlikely(TransactionIdIsValid(CheckXidAlive) && !bsysscan))
		elog(ERROR, "unexpected table_scan_sample_next_block call during logical decoding");
	return scan->rs_rd->rd_tableam->scan_sample_next_block(scan, scanstate);
}

/*
 * Fetch the next sample tuple into `slot` and return true if a visible tuple