equivclass.c

来自「postgresql8.3.4源码,开源数据库」· C语言 代码 · 共 1,975 行 · 第 1/5 页

		foreach(lc2, cur_ec->ec_members)
		{
			EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc2);

			/*
			 * If below an outer join, don't match constants: they're not as
			 * constant as they look.
			 */
			if (cur_ec->ec_below_outer_join &&
				cur_em->em_is_const)
				continue;

			if (expr_datatype == cur_em->em_datatype &&
				equal(expr, cur_em->em_expr))
				return cur_ec;	/* Match! */
		}
	}

	/*
	 * No match, so build a new single-member EC
	 *
	 * Here, we must be sure that we construct the EC in the right context. We
	 * can assume, however, that the passed expr is long-lived.
	 */
	oldcontext = MemoryContextSwitchTo(root->planner_cxt);

	newec = makeNode(EquivalenceClass);
	newec->ec_opfamilies = list_copy(opfamilies);
	newec->ec_members = NIL;
	newec->ec_sources = NIL;
	newec->ec_derives = NIL;
	newec->ec_relids = NULL;
	newec->ec_has_const = false;
	newec->ec_has_volatile = contain_volatile_functions((Node *) expr);
	newec->ec_below_outer_join = false;
	newec->ec_broken = false;
	newec->ec_sortref = sortref;
	newec->ec_merged = NULL;
	newem = add_eq_member(newec, expr, pull_varnos((Node *) expr),
						  false, expr_datatype);

	/*
	 * add_eq_member doesn't check for volatile functions, set-returning
	 * functions, or aggregates, but such could appear in sort expressions; so
	 * we have to check whether its const-marking was correct.
	 */
	if (newec->ec_has_const)
	{
		if (newec->ec_has_volatile ||
			expression_returns_set((Node *) expr) ||
			contain_agg_clause((Node *) expr))
		{
			newec->ec_has_const = false;
			newem->em_is_const = false;
		}
	}

	root->eq_classes = lappend(root->eq_classes, newec);

	MemoryContextSwitchTo(oldcontext);

	return newec;
}


/*
 * generate_base_implied_equalities
 *	  Generate any restriction clauses that we can deduce from equivalence
 *	  classes.
 *
 * When an EC contains pseudoconstants, our strategy is to generate
 * "member = const1" clauses where const1 is the first constant member, for
 * every other member (including other constants).	If we are able to do this
 * then we don't need any "var = var" comparisons because we've successfully
 * constrained all the vars at their points of creation.  If we fail to
 * generate any of these clauses due to lack of cross-type operators, we fall
 * back to the "ec_broken" strategy described below.  (XXX if there are
 * multiple constants of different types, it's possible that we might succeed
 * in forming all the required clauses if we started from a different const
 * member; but this seems a sufficiently hokey corner case to not be worth
 * spending lots of cycles on.)
 *
 * For ECs that contain no pseudoconstants, we generate derived clauses
 * "member1 = member2" for each pair of members belonging to the same base
 * relation (actually, if there are more than two for the same base relation,
 * we only need enough clauses to link each to each other).  This provides
 * the base case for the recursion: each row emitted by a base relation scan
 * will constrain all computable members of the EC to be equal.  As each
 * join path is formed, we'll add additional derived clauses on-the-fly
 * to maintain this invariant (see generate_join_implied_equalities).
 *
 * If the opfamilies used by the EC do not provide complete sets of cross-type
 * equality operators, it is possible that we will fail to generate a clause
 * that must be generated to maintain the invariant.  (An example: given
 * "WHERE a.x = b.y AND b.y = a.z", the scheme breaks down if we cannot
 * generate "a.x = a.z" as a restriction clause for A.)  In this case we mark
 * the EC "ec_broken" and fall back to regurgitating its original source
 * RestrictInfos at appropriate times.	We do not try to retract any derived
 * clauses already generated from the broken EC, so the resulting plan could
 * be poor due to bad selectivity estimates caused by redundant clauses.  But
 * the correct solution to that is to fix the opfamilies ...
 *
 * Equality clauses derived by this function are passed off to
 * process_implied_equality (in plan/initsplan.c) to be inserted into the
 * restrictinfo datastructures.  Note that this must be called after initial
 * scanning of the quals and before Path construction begins.
 *
 * We make no attempt to avoid generating duplicate RestrictInfos here: we
 * don't search ec_sources for matches, nor put the created RestrictInfos
 * into ec_derives.  Doing so would require some slightly ugly changes in
 * initsplan.c's API, and there's no real advantage, because the clauses
 * generated here can't duplicate anything we will generate for joins anyway.
 */
void
generate_base_implied_equalities(PlannerInfo *root)
{
	ListCell   *lc;
	Index		rti;

	foreach(lc, root->eq_classes)
	{
		EquivalenceClass *ec = (EquivalenceClass *) lfirst(lc);

		Assert(ec->ec_merged == NULL);	/* else shouldn't be in list */
		Assert(!ec->ec_broken); /* not yet anyway... */

		/* Single-member ECs won't generate any deductions */
		if (list_length(ec->ec_members) <= 1)
			continue;

		if (ec->ec_has_const)
			generate_base_implied_equalities_const(root, ec);
		else
			generate_base_implied_equalities_no_const(root, ec);

		/* Recover if we failed to generate required derived clauses */
		if (ec->ec_broken)
			generate_base_implied_equalities_broken(root, ec);
	}

	/*
	 * This is also a handy place to mark base rels (which should all exist by
	 * now) with flags showing whether they have pending eclass joins.
	 */
	for (rti = 1; rti < root->simple_rel_array_size; rti++)
	{
		RelOptInfo *brel = root->simple_rel_array[rti];

		if (brel == NULL)
			continue;

		brel->has_eclass_joins = has_relevant_eclass_joinclause(root, brel);
	}
}

/*
 * generate_base_implied_equalities when EC contains pseudoconstant(s)
 */
static void
generate_base_implied_equalities_const(PlannerInfo *root,
									   EquivalenceClass *ec)
{
	EquivalenceMember *const_em = NULL;
	ListCell   *lc;

	/*
	 * In the trivial case where we just had one "var = const" clause,
	 * push the original clause back into the main planner machinery.  There
	 * is nothing to be gained by doing it differently, and we save the
	 * effort to re-build and re-analyze an equality clause that will be
	 * exactly equivalent to the old one.
	 */
	if (list_length(ec->ec_members) == 2 &&
		list_length(ec->ec_sources) == 1)
	{
		RestrictInfo *restrictinfo = (RestrictInfo *) linitial(ec->ec_sources);

		if (bms_membership(restrictinfo->required_relids) != BMS_MULTIPLE)
		{
			distribute_restrictinfo_to_rels(root, restrictinfo);
			return;
		}
	}

	/* Find the constant member to use */
	foreach(lc, ec->ec_members)
	{
		EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc);

		if (cur_em->em_is_const)
		{
			const_em = cur_em;
			break;
		}
	}
	Assert(const_em != NULL);

	/* Generate a derived equality against each other member */
	foreach(lc, ec->ec_members)
	{
		EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc);
		Oid			eq_op;

		Assert(!cur_em->em_is_child);	/* no children yet */
		if (cur_em == const_em)
			continue;
		eq_op = select_equality_operator(ec,
										 cur_em->em_datatype,
										 const_em->em_datatype);
		if (!OidIsValid(eq_op))
		{
			/* failed... */
			ec->ec_broken = true;
			break;
		}
		process_implied_equality(root, eq_op,
								 cur_em->em_expr, const_em->em_expr,
								 ec->ec_relids,
								 ec->ec_below_outer_join,
								 cur_em->em_is_const);
	}
}

/*
 * generate_base_implied_equalities when EC contains no pseudoconstants
 */
static void
generate_base_implied_equalities_no_const(PlannerInfo *root,
										  EquivalenceClass *ec)
{
	EquivalenceMember **prev_ems;
	ListCell   *lc;

	/*
	 * We scan the EC members once and track the last-seen member for each
	 * base relation.  When we see another member of the same base relation,
	 * we generate "prev_mem = cur_mem".  This results in the minimum number
	 * of derived clauses, but it's possible that it will fail when a
	 * different ordering would succeed.  XXX FIXME: use a UNION-FIND
	 * algorithm similar to the way we build merged ECs.  (Use a list-of-lists
	 * for each rel.)
	 */
	prev_ems = (EquivalenceMember **)
		palloc0(root->simple_rel_array_size * sizeof(EquivalenceMember *));

	foreach(lc, ec->ec_members)
	{
		EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc);
		int			relid;

		Assert(!cur_em->em_is_child);	/* no children yet */
		if (bms_membership(cur_em->em_relids) != BMS_SINGLETON)
			continue;
		relid = bms_singleton_member(cur_em->em_relids);
		Assert(relid < root->simple_rel_array_size);

		if (prev_ems[relid] != NULL)
		{
			EquivalenceMember *prev_em = prev_ems[relid];
			Oid			eq_op;

			eq_op = select_equality_operator(ec,
											 prev_em->em_datatype,
											 cur_em->em_datatype);
			if (!OidIsValid(eq_op))
			{
				/* failed... */
				ec->ec_broken = true;
				break;
			}
			process_implied_equality(root, eq_op,
									 prev_em->em_expr, cur_em->em_expr,
									 ec->ec_relids,
									 ec->ec_below_outer_join,
									 false);
		}
		prev_ems[relid] = cur_em;
	}

	pfree(prev_ems);

	/*
	 * We also have to make sure that all the Vars used in the member clauses
	 * will be available at any join node we might try to reference them at.
	 * For the moment we force all the Vars to be available at all join nodes
	 * for this eclass.  Perhaps this could be improved by doing some
	 * pre-analysis of which members we prefer to join, but it's no worse than
	 * what happened in the pre-8.3 code.
	 */
	foreach(lc, ec->ec_members)
	{
		EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc);
		List	   *vars = pull_var_clause((Node *) cur_em->em_expr, false);

		add_vars_to_targetlist(root, vars, ec->ec_relids);
		list_free(vars);
	}
}

/*
 * generate_base_implied_equalities cleanup after failure
 *
 * What we must do here is push any zero- or one-relation source RestrictInfos
 * of the EC back into the main restrictinfo datastructures.  Multi-relation
 * clauses will be regurgitated later by generate_join_implied_equalities().
 * (We do it this way to maintain continuity with the case that ec_broken
 * becomes set only after we've gone up a join level or two.)
 */
static void
generate_base_implied_equalities_broken(PlannerInfo *root,
										EquivalenceClass *ec)
{
	ListCell   *lc;

	foreach(lc, ec->ec_sources)
	{
		RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(lc);

		if (bms_membership(restrictinfo->required_relids) != BMS_MULTIPLE)
			distribute_restrictinfo_to_rels(root, restrictinfo);
	}
}


/*
 * generate_join_implied_equalities
 *	  Generate any join clauses that we can deduce from equivalence classes.
 *
 * At a join node, we must enforce restriction clauses sufficient to ensure
 * that all equivalence-class members computable at that node are equal.
 * Since the set of clauses to enforce can vary depending on which subset
 * relations are the inputs, we have to compute this afresh for each join
 * path pair.  Hence a fresh List of RestrictInfo nodes is built and passed
 * back on each call.
 *
 * The results are sufficient for use in merge, hash, and plain nestloop join
 * methods.  We do not worry here about selecting clauses that are optimal
 * for use in a nestloop-with-inner-indexscan join, however.  indxpath.c makes
 * its own selections of clauses to use, and if the ones we pick here are
 * redundant with those, the extras will be eliminated in createplan.c.
 *
 * Because the same join clauses are likely to be needed multiple times as
 * we consider different join paths, we avoid generating multiple copies:
 * whenever we select a particular pair of EquivalenceMembers to join,
 * we check to see if the pair matches any original clause (in ec_sources)
 * or previously-built clause (in ec_derives).	This saves memory and allows
 * re-use of information cached in RestrictInfos.
 */
List *
generate_join_implied_equalities(PlannerInfo *root,
								 RelOptInfo *joinrel,
								 RelOptInfo *outer_rel,
								 RelOptInfo *inner_rel)
{
	List	   *result = NIL;
	ListCell   *lc;

	foreach(lc, root->eq_classes)
	{
		EquivalenceClass *ec = (EquivalenceClass *) lfirst(lc);
		List	   *sublist = NIL;

		/* ECs containing consts do not need any further enforcement */
		if (ec->ec_has_const)
			continue;

		/* Single-member ECs won't generate any deductions */
		if (list_length(ec->ec_members) <= 1)
			continue;

		/* We can quickly ignore any that don't overlap the join, too */
		if (!bms_overlap(ec->ec_relids, joinrel->relids))
			continue;

		if (!ec->ec_broken)
			sublist = generate_join_implied_equalities_normal(root,
															  ec,
															  joinrel,
															  outer_rel,
															  inner_rel);

		/* Recover if we failed to generate required derived clauses */
		if (ec->ec_broken)
			sublist = generate_join_implied_equalities_broken(root,
															  ec,
															  joinrel,
															  outer_rel,
															  inner_rel);

		result = list_concat(result, sublist);
	}

	return result;
}