planner: introduce Cond-FD to maintain null-constraint FD (#34147)

close #34148

planner/core/logical_plans.go

@@ -965,19 +965,6 @@ func (p *LogicalSelection) ExtractFD() *fd.FDSet {
 	// extract equivalence cols.
 	equivUniqueIDs := extractEquivalenceCols(p.Conditions, p.SCtx(), fds)
 
-	// after left join, according to rule 3.3.3, it may create a lax FD from inner equivalence
-	// cols pointing to outer equivalence cols.  eg: t left join t1 on t.a = t1.b, leading a
-	// lax FD from t1.b ~> t.a, this lax attribute is coming from supplied null value to all
-	// left rows, once there is a null-refusing predicate on the inner side on upper layer, this
-	// can be equivalence again. (the outer rows left are all coming from equal matching)
-	//
-	// why not just makeNotNull of them, because even a non-equiv-related inner col can also
-	// refuse supplied null values.
-	if fds.Rule333Equiv.InnerCols.Len() != 0 && notnullColsUniqueIDs.Intersects(fds.Rule333Equiv.InnerCols) {
-		// restore/re-strength FDs from rule 333
-		fds.MakeRestoreRule333()
-	}
-
 	// apply operator's characteristic's FD setting.
 	fds.MakeNotNull(notnullColsUniqueIDs)
 	fds.AddConstants(constUniqueIDs)

planner/funcdep/doc.go

@@ -0,0 +1,180 @@
+// Copyright 2022 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package funcdep
+
+// Theory to Practice
+//
+// For more rigorous examination of functional dependencies and their
+// interaction with various SQL operators, see the following Master's Thesis:
+//
+//   Norman Paulley, Glenn. (2000).
+//   Exploiting Functional Dependence in Query Optimization.
+//   https://cs.uwaterloo.ca/research/tr/2000/11/CS-2000-11.thesis.pdf
+
+// TODO: Add the RFC design.
+
+// NOTE 1.
+// when handling Lax FD, we don't care the null value in the dependency, which means
+// as long as null-attribute coverage of the determinant can make a Lax FD as strict one.
+
+// The definition of "lax" used in the paper differs from the definition used by this
+// library. For a lax dependency A~~>B, the paper allows this set of rows:
+//
+//	a  b
+//	-------
+//	1  1
+//	1  NULL
+//
+//	This alternate definition is briefly covered in section 2.5.3.2 of the paper (see definition
+//	2.19). The reason for this change is to allow a lax dependency to be upgraded to a strict
+//	dependency more readily, needing only the determinant columns to be not-null rather than
+//  both determinant and dependant columns.
+//
+// This is on the condition that, for definite values of determinant of a Lax FD, it won't
+// have two same definite dependant value. That's true, because there is no way can derive
+// to this kind of FD.
+//
+// Even in our implementation of outer join, the only way to produce duplicate definite
+// determinant is the join predicate. But for now, we only maintain the equivalence and
+// some strict FD of it.
+//
+//   t(a,b) left join t1(c,d,e) on t.a = t1.c and b=1
+//  a  b  |  c     d     e
+//  ------+----------------
+//  1  1  |  1    NULL   1
+//  1  2  | NULL  NULL  NULL
+//  2  1  | NULL  NULL  NULL
+//
+// Actually it's possible, the lax FD {a} -> {c} can be derived but not that useful. we only
+// maintain the {c} ~> {a} for existence after outer join. Besides, there two Cond-FD should
+// be preserved waiting for be visible again once with the null-reject on the condition of
+// null constraint columns. (see below)
+//
+// NOTE 2.
+// When handle outer join, it won't produce lax FD with duplicate definite determinant values and
+// different dependency values.
+//
+// In implementation，we come across some lax FD dependent on null-reject of some other cols. For
+// example.
+//   t(a,b) left join t1(c,d,e) on t.a = t1.c and b=1
+//  a  b  |  c     d     e
+//  ------+----------------
+//  1  1  |  1    NULL   1
+//  1  2  | NULL  NULL  NULL
+//  2  1  | NULL  NULL  NULL
+//
+// here constant FD {} -> {b} won't be existed after the outer join is done. Notice null-constraint
+// {c,d,e} -| {c,d,e}, this FD should be preserved and will be visible again when some null-reject
+// predicate take effect on the null-constraint cols.
+//
+// It's same for strict equivalence {t.a} = {t1.c}. Notice there are no lax equivalence here, because
+// left side couldn't be guaranteed to be definite or null. like a=2 here. Let's collect all of this
+// on-condition FD down, correspondent with a null-constraints column set, name it as Cond-FD.
+//
+// lax equivalencies are theoretically possible, but it won't be constructed from an outer join unless
+// t already has a constant FD in column `a` here before outer join take a run. So the lax equivalence
+// has some pre-conditions as you see, and it couldn't cover the case shown above. Let us do it like a
+// Cond-FD does.
+//
+// The FD constructed from the join predicate should be considered as Cond-FD. Here like equivalence of
+// {a} == {c} and constant FD {b} = 1 (if the join condition is e=1, it's here too). We can say that for
+// every matched row, this FDs is valid, while for the other rows, the inner side are supplied of null
+// rows. So this FDs are stored as ncEdges with nc condition of all inner table cols.
+//
+// We introduced invisible FD with null-constraint column to solve the problem above named as Cond-FD.
+// For multi embedded left join, we take the following case as an example.
+//    a,b         c,d,e
+// 	-----------+-----------
+//   1    2    |    1  1  1
+// 	 2    2    |
+//  -----------+-----------
+//
+//  left join on (a=c) res:
+//   a   b    c     e     e
+//  -------------------------
+//   1   2    1     1     1
+//   2   2 +- null null  null -+
+//         |                   |
+//         +-------------------+
+//                              \
+//                               \
+//  the Cond-FD are < a=c with {c,d,e} > the latter is as null constraint cols
+//
+//    e,f
+//  -----------------------
+//   1   2
+//   2   2
+//   3   3
+//  -----------------------
+//
+//  left join on (e=a) res:
+//   e   f    a     b      c    d    e
+//  -----------------------------------
+//   1   2    1     2      1    1    1
+//   2   2    2     2  +- null null null --+---------------> Cond-FD are <a=c with {c,d,e}> still exists.
+//   3   3 +-null null |  null null null   |---+
+//         |           +-------------------+   |
+//         +-----------------------------------+-----------> New Cond-FD are <e=a with {a,b,c,d,e}> occurs.
+//
+//
+// the old Cond-FD with null constraint columns set {c,d,e} is preserved cause new appended cols are all null too.
+// the new Cond-FD with null constraint columns set {a,b,c,d,e} are also meaningful, even if the null-reject column
+// is one of {c,d,e} which may reduce one of the matched row out of the result, the equivalence {a}={e} still exist.
+//
+// Provide that the result of the first left join is like:
+//  left join on (a=c) res:
+//   a     b    c     e     e
+//  ---------------------------
+//   1     2    1     1     1
+//   null  2  null  null  null
+//
+//  THEN: left join on (e=a) res:
+//   e   f    a     b     c    d    e
+//  ---------------------------------
+//   1   2    1     2     1    1    1
+//   2   2    null null null null null
+//   3   3    null null null null null
+//
+//  Even like that, the case of old Cond-FD and new Cond-FD are existed too. Seems the null-constraint column set of
+//  old Cond-FD {c,d,e} can be expanded as {a,b,c,d,e} visually, but we couldn't derive the inference of the join predicate
+//  (e=a). The null-reject of column `a` couldn't bring the visibility to the old Cond-FD theoretically, it just happened
+//  to refuse that row with a null value in column a.
+//
+// Think about adding one more row in first left join result.
+//
+//  left join on (a=c) res:
+//   a     b    c     e     e
+//  ---------------------------
+//   1     2    1     1     1
+//   null  2  null  null  null
+//   3     3  null  null  null
+//
+//  THEN: left join on (e=a) res:
+//   e   f    a     b     c    d    e
+//  ---------------------------------
+//   1   2    1     2     1    1    1
+//   2   2    null null null null null
+//   3   3    3     3   null null null
+//
+//  Conclusion:
+//  As you see that's right we couldn't derive the inference of the join predicate (e=a) to expand old Cond-FD's nc
+//  {c,d,e} as {a,b,c,d,e}. So the rule for Cond-FD is quite simple, just keep the old ncEdge from right, appending
+//  the new ncEdges in current left join.
+//
+//  If the first left join result is in the outer side of the second left join, just keep the ncEdge from left as well,
+//  appending the new ncEdges in current left join.
+//
+//  For a inner join, both side of the join result won't be appended with null-supplied rows, so we can simply collect
+//  the ncEdges from both join side together.