Function RelationGetIndexPredicate

src/backend/utils/cache/relcache.c:5289–5346

* RelationGetIndexPredicate -- get the index predicate for an index * * We cache the result of transforming pg_index.indpred into an implicit-AND * node tree (suitable for use in planning). * If the rel is not an index or has no predicate, we return NIL. * Otherwise, the returned tree is copied into the caller's memory context. * (We don't want to return a pointer to the relcache copy, since

 * disappear due to relcache invalidation.)
 */
List *
RelationGetIndexPredicate(Relation relation)
{
	List	   *result;
	Datum		predDatum;
	bool		isnull;
	char	   *predString;
	MemoryContext oldcxt;

	/* Quick exit if we already computed the result. */
	if (relation->rd_indpred)
		return copyObject(relation->rd_indpred);

	/* Quick exit if there is nothing to do. */
	if (relation->rd_indextuple == NULL ||
		heap_attisnull(relation->rd_indextuple, Anum_pg_index_indpred, NULL))
		return NIL;

	/*
	 * We build the tree we intend to return in the caller's context. After
	 * successfully completing the work, we copy it into the relcache entry.
	 * This avoids problems if we get some sort of error partway through.
	 */
	predDatum = heap_getattr(relation->rd_indextuple,
							 Anum_pg_index_indpred,
							 GetPgIndexDescriptor(),
							 &isnull);
	Assert(!isnull);
	predString = TextDatumGetCString(predDatum);
	result = (List *) stringToNode(predString);
	pfree(predString);

	/*
	 * Run the expression through const-simplification and canonicalization.
	 * This is not just an optimization, but is necessary, because the planner
	 * will be comparing it to similarly-processed qual clauses, and may fail
	 * to detect valid matches without this.  This must match the processing
	 * done to qual clauses in preprocess_expression()!  (We can skip the
	 * stuff involving subqueries, however, since we don't allow any in index
	 * predicates.)
	 */
	result = (List *) eval_const_expressions(NULL, (Node *) result);

	result = (List *) canonicalize_qual((Expr *) result, false);

	/* Also convert to implicit-AND format */
	result = make_ands_implicit((Expr *) result);

	/* May as well fix opfuncids too */
	fix_opfuncids((Node *) result);

	/* Now save a copy of the completed tree in the relcache entry. */
	oldcxt = MemoryContextSwitchTo(relation->rd_indexcxt);
	relation->rd_indpred = copyObject(result);
	MemoryContextSwitchTo(oldcxt);

	return result;
}