Function set_tablesample_rel_pathlist

src/backend/optimizer/path/allpaths.c:1145–1187

* set_tablesample_rel_pathlist * Build access paths for a sampled relation */

 *	  Build access paths for a sampled relation
 */
static void
set_tablesample_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
{
	Relids		required_outer;
	Path	   *path;

	/*
	 * We don't support pushing join clauses into the quals of a samplescan,
	 * but it could still have required parameterization due to LATERAL refs
	 * in its tlist or TABLESAMPLE arguments.
	 */
	required_outer = rel->lateral_relids;

	/* Consider sampled scan */
	path = create_samplescan_path(root, rel, required_outer);

	/*
	 * If the sampling method does not support repeatable scans, we must avoid
	 * plans that would scan the rel multiple times.  Ideally, we'd simply
	 * avoid putting the rel on the inside of a nestloop join; but adding such
	 * a consideration to the planner seems like a great deal of complication
	 * to support an uncommon usage of second-rate sampling methods.  Instead,
	 * if there is a risk that the query might perform an unsafe join, just
	 * wrap the SampleScan in a Materialize node.  We can check for joins by
	 * counting the membership of all_baserels (note that this correctly
	 * counts inheritance trees as single rels).  If we're inside a subquery,
	 * we can't easily check whether a join might occur in the outer query, so
	 * just assume one is possible.
	 *
	 * GetTsmRoutine is relatively expensive compared to the other tests here,
	 * so check repeatable_across_scans last, even though that's a bit odd.
	 */
	if ((root->query_level > 1 ||
		 bms_membership(root->all_baserels) != BMS_SINGLETON) &&
		!(GetTsmRoutine(rte->tablesample->tsmhandler)->repeatable_across_scans))
	{
		path = (Path *) create_material_path(rel, path);
	}

	add_path(rel, path, root);

	/* For the moment, at least, there are no other paths to consider */
}

/*
 * set_foreign_size