Hans Zeller
on 27 Oct 16
[TRAFODION-2317] Infrastructure for common subexpressions
This is a first set of changes to allow us to make use of CTEs
(Common Table Expre… Show more
[TRAFODION-2317] Infrastructure for common subexpressions

This is a first set of changes to allow us to make use of CTEs

(Common Table Expressions) declared in WITH clauses and to create

a temp table for them that is then read multiple times in the query.

This also includes a fix for

[TRAFODION-2280] Need to remove salt columns from uniqueness constraints

Summary of changes:

- Adding a unique statement number in CmpContext

- Moving the execHiveSQL method from the OSIM code to CmpContext

- Adding a list of common subexpressions and their references

 to CmpStatement

- Adding the ability to the Hive Truncate operator to drop the

 table when the TCB gets deallocated

- Adding the ability to the HDFS scan to compute scan ranges at

 runtime. Those are usually determined in the compiler. This is

 only supported for simple, non-partitioned, delimited tables.

 We need this because we populate the temp table and read from

 in in the same statement, without the option of compiling

 after we inserted into the temp table.

- Special handling in the MapValueIds node of common subexpressions.

 See the comment in MapValueId::preCodeGen().

- Moved the binder code to create a FastExtract node into a new

 method FastExtract::makeFastExtractTree(), to be able to call

 it from another place.

- MapValueIds no longer looks at the "used by MVQR flag" to determine

 the method for VEGRewrite. Instead it checks whether a list of

 values has been provided to do this.

- Adding a new method, RelExpr::prepareMeForCSESharing, that is

 kind of an "unnormalizer", undoing some of the normalizer


- Implementing the steps for common subexpression materializations

 described below.

- Adding the ability to suppress the Hive timestamp modification

 check when truncating a Hive table

- Adding an optimizer rule to eliminate CommonSubExprRef nodes.

 These nodes should not normally survive past the SQO phase, but

 if the SQO phase gets interrupted by an exception, that could

 happen, since we then fall back to a copy of the tree before

 SQO. In the future, we can consider cost-based decision on

 what to do with common subexpressions.

- Adding CommonSubExprRef nodes in the parser whenever we expand

 a CTE reference.

- Adding cleanup code to the "cleanup obsolete volatile tables"

 command that removes obsolete Hive tables used for common


Other changes contained in this change set:

- Optimization for empty scans, like select * from t where 1=0

 This now generates a cardinality constraint with 0 rows, which

 can be used later to eliminate parts of a tree.

 (file OptLogRelExpr.cpp)

- [TRAFODION-2280] Need to remove salt columns from uniqueness

 constraints generated on salted tables.

 (file OptLogRelExpr.cpp)

- Got rid of the now meaningless "seamonster" display in EXPLAIN.

 (file GenExplain.cpp and misc. expected files)

- Suppress display of "zombies" in the cstat command. Otherwise,

 these zombies (marked as <defunct>) prevent Trafodion from

 starting, because they are incorrectly considered "orphan"

 processes. This could require a reboot when no reboot is necessary.

 (file core/sqf/sql/scripts/pstat)

Incomplete list of things left to be done:

- TRAFODION-2316: Hive temp tables are not secure. Use volatile

                 tables instead.

- TRAFODION-2315: Add heuristics to decide when to use the temp table


- TRAFODION-2320: Make subquery unnesting work with common subexpressions.

Generated Plans


The resulting query plan for a query Q with n common

subexpressions CSE1 ... CSEn looks like this:






                      /        \

                   Union        Q

                   /   \

                 ...    CTn



              /   \

            CT1   CT2

Each of the CTi variables looks like the following, an



         /        \

     Truncate  FastExtract TEMPi

       TEMPi        |


The original query Q has the common subexpressions replaced

with the following:



            scan TEMPi

Here is a simple query and its explain:

prepare s from

with cse1 as (select d_date_sk, d_date, d_year, d_dow, d_moy from date_dim)

select x.d_year, y.d_date

from cse1 x join cse1 y on x.d_date_sk = y.d_date_sk

where x.d_moy = 3;

>>explain options 'f' s;

LC   RC   OP   OPERATOR              OPT       DESCRIPTION           CARD

---- ---- ---- --------------------  --------  --------------------  ---------

11   .    12   root                                                  1.46E+005

5    10   11   blocked_union                                         1.46E+005

7    9    10   merge_join                                            7.30E+004

8    .    9    sort                                                  1.00E+002

.    .    8    hive_scan                       CSE_TEMP_CSE1_MXID11  1.00E+002

6    .    7    sort                                                  5.00E+001

.    .    6    hive_scan                       CSE_TEMP_CSE1_MXID11  5.00E+001

1    4    5    blocked_union                                         7.30E+004

2    .    4    hive_insert                     CSE_TEMP_CSE1_MXID11  7.30E+004

.    .    2    hive_scan                       DATE_DIM              7.30E+004

.    .    1    hive_truncate                                         1.00E+000

--- SQL operation complete.


CQDs to control common subexpressions


CSE_FOR_WITH is the master switch.

CQD                    Value      Default  Behavior

---------------------  ---------  -------  ---------------------------------------

CSE_FOR_WITH             OFF        Y      No change

                        ON                Insert a CommonSubExprRef node in the

                                          tree whenever we reference a CTE

                                          (table defined in a WITH clause)

CSE_USE_TEMP             OFF        Y      Disable creation of temp tables

                                          for common subexpressions

                        SYSTEM            Same as OFF for now

                        ON                Always create a temp table for

                                          common subexpressions where possible

CSE_DEBUG_WARNINGS       OFF        Y      No change

                        ON                Emit diagnostic warnings that show why

                                          we didn't create temp tables for

                                          common subexpressions

CSE_CLEANUP_HIVE_TABLES  OFF        Y      No change

                        ON                Cleanup Hive tables used for CSEs with

                                          the "cleanup obsolete volatile tables"


CommonSubExprRef relational operators


This is a new RelExpr class that is introduced. It marks the common

subexpressions in a RelExpr tree. This operator remembers the name of

a common subexpression (e.g. the name used in the WITH clause).

Multiple such operators can reference to the same name. Each of

these references has a copy of the tree.

Right now, these operators are created in the parser when we expand a

CTE (Common Table Expression), declared in a WITH clause.  If the CTE

is referenced only once, then the CommonSubExprRef operator is removed

in the binder - it also doesn't live up to its name in this case.

The remaining CommonSubExprRef operators keep track of various changes

to their child trees, during the binder and normalizer phases.  In

particular, it tracks which predicates are pushed down into the child

tree and which outputs are eliminated.

The CmpStatement object keeps a global list of all the

CommonSubExprRef operators in a statement, so the individual operators

have a way to communicate with their siblings:

- A statement can have zero or more named common subexpressions.

- Each reference to a common subexpression is marked in the RelExpr

 tree with a CommonSubExprRef node.

- In the binder and normalizer, common subexpressions are expanded,

 meaning that multiple copies of them exist, one copy per


- Common subexpressions can reference other common subexpressions,

 so they, together with the main query, for a DAG (directed

 acyclic graph) of dependencies.

- Note that CTEs declared in a WITH clause but not referenced are

 ignored and are not part of the query tree.

In the semantic query optimization phase (SQO), the current code makes

a heuristic decision what to do with common subexpressions - to

evaluate them multiple times (expand) or to create a temporary table

once and read that table multiple times.

If we decide to expand, the action is simple: Remove the

CommonSubExprRef operator from the tree and replace it with its child.

If we decide to create a temp table, things become much more difficult.

We need to do several steps:

- Pick one of the child trees of the CommonSubExprRefs as the one to


- Undo any normalization steps that aren't compatible with the other

 CommonSubExprRefs. That means pulling out predicates that are not

 common among the references and adding back outputs that are

 required by other references. If that process fails, we fall back

 and expand the expressions.

- Create a temp table tmp.

- Prepare an INSERT OVERWRITE TABLE tmp SELECT * FROM cse tree

 that materializes the common subexpression in a table.

- Replace the CommonSubExprRef nodes with scans of the temp table.

- Hook up the insert query tree with the main query, such that it

 is executed before the main query starts.

Temporary tables


At this time, temporary tables are created as Hive tables, with a

fabricated, unique name, including the session id, a unique statement

number within the session, and a unique identifier of the common

subexpression within the statement. The temporary table is created at

compile time. The query plan contains an operator to truncate the

table before populating it. The "temporary" Hive table is dropped when

the executor TCB is deallocated.

Several issues are remaining with this approach:

- If the process exits before executing and deallocating the statement,

 the Hive table is not cleaned up.

 Solution (TBD): Clean up these tables like we clean up left-over

 volatile tables. Both are identified by the session id.

- If the executor runs into memory pressure and deallocates the TCB,

 then allocates it again at a later time, the temp table is no longer


 Solution (TBD): Use AQR to recompile the query and create a new table.

- Query cache: If we cache a query, multiple queries may end up with

 the same temporary table. This works ok as long as these queries are

 executed serially, but it fails if both queries are executed at the

 same time (e.g. open two cursors and fetch from both, alternating).

 Solution (TBD): Add a CQD that disables caching queries with temp tables

 for common subexpressions.

In the future we also want to support volatile tables. However, those also

have issues:

- Volatile tables aren't cleaned up until the session ends. If we run

 many statements with common subexpressions, that is undesirable. So,

 we have a similar cleanup problem as with Hive tables.

- Volatile tables take a relatively long time to create.

- Insert and scan rates on volatile Trafodion tables are slower than

 those on Hive tables.

To-do items are marked with "Todo: CSE: " in the code.

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