WL#3750: Subquery optimization: Semijoin: First-match strategy

Affects: Server-6.0   —   Status: Complete   —   Priority: Low

This is a subtask of WL#2980 "Subquery optimization: Semijoin".

This WL entry covers implementation of First-match (formerly known as "Join
ShortCutting) semi-join execution strategy.
1. The idea of FirstMatch strategy
2. What join orders can be handled
2.1 Why inner tables cannot be before the outer
2.2 Interleaving with "unrelated" tables
2.3 Acceptable join orders for FirstMatch strategy
3. Use of FirstMatch together with DuplicateWeedout
3. FirstMatch and block NL-join
4. Implementation of FirstMatch
4.1 Accounting for FirstMatch in join optimizer
4.2 Executioner
4.3 EXPLAIN output


1. The idea of FirstMatch strategy

Suppose we have semi-join, and the join order is 

   ot1, ot2, ... otN, it1, it2, ... itK

(the naming convention is same as in from WL#3741: ot{i} is a table from the
parent select, it{i} is a table from within a semi-join, i.e. from the

Let us start executing NL-Join algorithm for this join order. As soon as we've
got a First Match, i.e. full row combination (ot1.row, .... itK.row), we 
return (or short-cut) to the last outer table otN, and continue scanning from 

It is apparent that we will still enumerate all (ot1.row, ..., onN.row) row
combinations. It is also apparent that the short-cutting will prevent
generation of any INNER-UNIQ-REQ (see WL#2980 for what it is) violations.

2. What join orders can be handled

As shown in previous section, FirstMatch strategy can be used when all outer
tables are followed by all inner tables.

2.1 Why inner tables cannot be before the outer

If the tables are intermixed, we may run into a problem as demonstrated in the
following example:

Consider a semijoin

  (ot1, ot2) SJ it1 ON sj_cond

Let the join order be ot1, it1, ot2. Let's start running the NL-join. Soon
we'll have the state as demonstrated on this swimlane diagram:

   ot1    |  it1     |  ot2
          |          |
 ot1.row1 | it1.row1 |  ot2.row1 
          |          |  ot2.row2
          |          |  <eof>(*)<---  we are here 

We've found two matching (i.e. result) tuples: 

 1. (ot1.row1, it1.row1, ot2.row1)
 2. (ot1.row1, it1.row1, ot2.row2)

Now there are no more matching records in ot2 and we must return.

If we return to it1, then 
  we may find a row it1.row2 and then proceed to generate 
  (ot1.row1, it1.row2, ot2.row[1|2]), which will violate the 

If we return to ot1, then 
  we will miss row combinations with ot1.row, like row combination:
  (ot1.row1, it1.somerow, ot2.row3).

In general case, we'll need to keep track of which rows from ot2 we've
returned, which requires O(#examined_rows(ot2)) and is out of scope of this
WL entry.

2.2 Interleaving with "unrelated" tables

Suppose there are outer tables wrt which the subquery is not correlated, and
also those tables are not referred to from sj_cond. We'll call those tables 
"unrelated" and denote them as ntX.

We can have unrelated tables interleaved with subquery tables if we use a
modified jump-out strategy:
 * When we get a matching (it1, ... itN) row combination, we jump back to
   last table ntX

 * When we get EOF on some table ntX, we jump to the previous outer table 
   (either related or unrelated)
Access/selection conditions of unrelated tables don't depend on what are the
current rows of the inner tables and our jumps don't make us skip any outer
table accesses, so we will not miss any row combinations of outer tables.

Now let's see what we have for INNER-UNIQ-REQ. Suppose we've reached the last
inner table and got a matching row combination. The jump-out rules require 
that we jump back until we get a new row for some outer table (related 
or unrelated). When we get that new row we'll have a new row combination of
outer tables and will have lost any chance to generate INNER-UNIQ-REQ
violation for row combination we've previously had.

2.3 Acceptable join orders for FirstMatch strategy
Putting the above considerations together, we get this criterion:

   Denoting the 
     - outer tables that are not referenced from subquery predicate as ntX
     - outer tables that are referenced as otX
     - inner tables as itX
   The acceptable join orders are those that match this BNF pattern:

     (otX | ntX)* , (itX | ntX)*

   This pattern will be referred to as FirstMatchPattern.

A duplicate-producing range can be handled by FirstMatch strategy if it
matches the FirstMatchPattern.

3. Use of FirstMatch together with DuplicateWeedout

For a fixed join order and a duplicate-producing range within it, FirstMatch 
strategy is cheaper than DuplicateWeedout because 

  * It "takes shortcuts" and so eliminates fewer rows than DuplicateWeedout.
  * It doesn't have the costs of using temptable.

For this reason, FirstMatch strategy will preferred over DuplicateWeedout
when it is applicable.

There are cases where FirstMatch strategy cannot eliminate duplicates itself,
hence we use DuplicateWeedout, and use the approach from the FirstMatch 
strategy for additional speedup.

Illustrating it with an example: Suppose we have a join [sub]order that can't
be served by FirstMatch strategy:

    it1  ot1  it2

However, once we got a matching row combination of

   (it1.rowX, ot1.rowX, it2.rowX)

we don't have to enumerate row combinations like 

   (it1.rowX, ot1.rowX, it2.*)

and can return to enumerating the last outer table, in our example ot1. When 
we generalize this, we get the following rule:

  If a duplicate-producing range has a suffix of (itX | ntX)*, then we can set
  up FirstMatch-type shortcutting within that suffix.

The validity of this approach is proved in the same way as validity of
FirstMatch and DuplicateWeedout are proved.

Addition of short-cutting does not allow to make any improvements to the way 
DuplicateWeedout strategy works. That is, the temptable tuple remains the 
same, temptable flush and weedout-check remain at the same locations. 

The shortcuts will only reduce the number of times the weedout-check will be

4. FirstMatch and block NL-join

For the same reasons as in WL#3741, FirstMatch strategy cannot be used when
one of the tables in the duplicate-generating range (concept defined in 
WL#3741) uses Block NL-join.

FirstMatch can be used without any adjustments if Block NL-join is used by
some table that is outside of the duplicate-generating range.

5. Implementation of FirstMatch

5.1 Accounting for FirstMatch in join optimizer

In scope of this WL use of FirstMatch will not be taken into account in join

We'll follow the approach of WL#3741: first choose the join order and then 
determine how semi-join duplicates should be eliminated. This will be done
according to the following rule:

  for each duplicate-generating range R 
    if (a table in R uses Block NL-join)
      Remove all DuplicateWeedout strategies we've set up for \
        the preceding ranges;
    else if (R matches FirstMatchPattern)
      Setup R to be handled with FirstMatch pattern by adding \
        appropriate shortcutting calls;
      setup_combined(R, first_table(R));

  setup_combined(R, first_tbl)
    Setup R to be handled with DuplicateWeedout strategy;
    S = range_suffix(R, first_tbl, last_table(R));
    if (S has a suffix that matches FirstMatchPattern)
      Add appropriate shortcutting calls in the suffix;

5.2 Executioner

FirstMatch shortcutting code has already been added in WL#3741.

5.3 EXPLAIN output

Use of FirstMatch will be indicated in the same way as it is done in WL#3741,
the appropriate tables will have

in the Extra column.
1. Implementation
1.1 Setup, plan refinement stage
1.2 NL-Join algorithm fixes 
2. EXPLAIN output
3. Schedule

1. Implementation

1.1 Setup, plan refinement stage
The setup will be done in setup_semijoin_dups_elimination(), a WL#3741
function that sets up use of temporary tables.

We'll be able to set up all of the duplication elimination in one
forward pass over the join order (as it is done in WL#3741).

(Prepared Statements are not a concern for this WL because all the used
structures are per-execution, and are pointed from class JOIN which is
per-execution too)

1.2 NL-Join algorithm fixes 
WL#3741 already included NL-join fixes neded for FirstMatch strategy.
For this WL we'll need to make sure that a "jump out" and a "temptable 
weedout" on the same table are performed correctly.

2. EXPLAIN output
A small change is needed to take into account that "End temporary" 
and "FirstMatch(tableX)" may occur at the same row.

3. Schedule
There will be only one milestone, the entire WL entry