hints on hints - oracle scratchpad | just another oracle · 4 hints on hints 7 / 78 jonathan...

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Hints (definition 1)

on

Hints (definition 2)

Jonathan Lewiswww.jlcomp.demon.co.uk

jonathanlewis.wordpress.com

Hints on Hints

2 / 78

Jonathan Lewis

© 2008 - 2009

Who am I ?

Independent Consultant.

23+ years in IT20+ using Oracle

Strategy, Design, ReviewBriefings, SeminarsTrouble-shooting

www.jlcomp.demon.co.ukjonathanlewis.wordpress.com

One of the directors of the UKOUGMember of the Oak Table Network.Oracle Author of the year 2006“Select” Editor’s choice 2007

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Jonathan Lewis

© 2008 - 2009

Highlights

The rules of hinting

What are hints.

Query Blocks

The meaning of hints

Strategic hints

How to get lucky

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

Rules of hinting

• Don’t

– Unless you really have to

• If you do hint, stick to the ‘strategic’ hints

– and assume you’ve got it wrong

• Test hinted SQL on every patch or upgrade

• Test it after every piece of associated DDL

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Jonathan Lewis

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Effects of hints

• Modify the optimizer’s choice of actions

– E.g. /*+ use_nl() */ /*+ no_merge() */

• Change the optimizer arithmetic

– E.g. /*+ cardinality() */ /*+ opt_estimate() */

• A few special cases (run-time / feature)

– E.g. /*+ append */ /*+ no_qkn_buff */

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Jonathan Lewis

© 2008 - 2009

Dangers of hints

• Oracle MUST obey your hints

– Unless you’ve got the syntax wrong

– Or you’ve got the spelling wrong

– Or you’ve got the context wrong

– Or the hint is illegal

– Or you have contradictory hints in one statement

• The hints are very badly documented !

– There are 236 hints in 11.1.0.6 (how many are documented ?)

– select * from v$sql_hint

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Jonathan Lewis

© 2008 - 2009

Ignoring hints – 1

create table t1(

n1 number,

n2 number,

n3 number,

constraint t1p primary key(n1),

constraint t1u unique (n2)

);

create bitmap index t1b on t1(n3);

How many ways could the optimizer choose to execute:

select count(*) from t1;

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Jonathan Lewis

© 2008 - 2009


select /*+ full (t1) */ count(*) from t1;

TABLE ACCESS (FULL) OF 'T1'

select /*+ index (t1,t1p) */ count(*) from t1;

INDEX (FULL SCAN) OF 'T1P' (UNIQUE)

select /*+ index_ffs(t1,t1p) */ count(*) from t1;

INDEX (FAST FULL SCAN) OF 'T1P' (UNIQUE)

select /*+ index (t1,t1b) */ count(*) from t1;

BITMAP INDEX (FULL SCAN) OF 'T1B'

select /*+ index_ffs(t1,t1b) */ count(*) from t1;

BITMAP INDEX (FAST FULL SCAN) OF 'T1B' -- not 8.1

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Jonathan Lewis

© 2008 - 2009


Is the optimizer ignoring hints ?

select /*+ index(t1,t1u) */ count(*) from t1;

BITMAP INDEX (FAST FULL SCAN) OF 'T1B'

select /*+ index(t1,t1u) */ count(*) from t1

where n2 is not null;

INDEX (FULL SCAN) OF 'T1U' (UNIQUE)

alter table t1 modify n2 not null;

select /*+ index(t1,t1u) */ count(*) from t1

INDEX (FULL SCAN) OF 'T1U' (UNIQUE)

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Jonathan Lewis

© 2008 - 2009


alter session set "_optimizer_ignore_hints"=true;

/*+ IGNORE_OPTIM_EMBEDDED_HINTS */

And from one 10053 trace file I got:

Trying or-Expansion on query block SEL$1 (#0)

USE_CONCAT hint was ignored.

atom_hint=(@=221EAF74 err=0 resol=1 used=1

token=921 org=1 lvl=2 txt=USE_CONCAT (8) )

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Jonathan Lewis

© 2008 - 2009


select /*+ use_concat */

small_vc

from t1

where id = 100 or id = 500;

Execution plan 8i and early 9i style.

SELECT STATEMENT Optimizer=ALL_ROWS (Cost=4 Card=2 Bytes=32)

CONCATENATION

TABLE ACCESS (BY INDEX ROWID) OF 'T1' (Cost=2 Card=1 Bytes=16)

INDEX (RANGE SCAN) OF 'T1_I1' (NON-UNIQUE) (Cost=1 Card=1)

TABLE ACCESS (BY INDEX ROWID) OF 'T1' (Cost=2 Card=1 Bytes=16)

INDEX (RANGE SCAN) OF 'T1_I1' (NON-UNIQUE) (Cost=1 Card=1)

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Jonathan Lewis

© 2008 - 2009


select /*+ use_concat */

small_vc

from t1

where id = 100 or id = 500;

Execution plan 10g .

| Id | Operation | Name | Rows | Bytes | Cost |

| 0 | SELECT STATEMENT | | 2 | 32 | 4 |

| 1 | INLIST ITERATOR | | | | |

| 2 | TABLE ACCESS BY INDEX ROWID| T1 | 2 | 32 | 4 |

|* 3 | INDEX RANGE SCAN | T1_I1 | 2 | | 3 |

Predicate Information (identified by operation id):

3 - access("ID"=100 OR "ID"=500)

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Jonathan Lewis

© 2008 - 2009

Nobody ignores me !

select

/*+

begin_outline_data

qb_name(main)

use_concat(@main)

outline(@main) outline_leaf(@main)

outline_leaf(@main_1) outline_leaf(@main_2)

index_rs_asc(@main_1 t1@main(t1.id))

index_rs_asc(@main_2 t1@main_2(t1.id))

end_outline_data

*/

small_vc

from t1

where id = 100 or id = 500; use_concat(1) also worked – all by itself

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Jonathan Lewis

© 2008 - 2009


Execution plan 10g .



| 1 | CONCATENATION | | | | |





Query Block Name / Object Alias (identified by operation id):

1 - MAIN

2 - MAIN_1 / T1@MAIN

3 - MAIN_1 / T1@MAIN

4 - MAIN_2 / T1@MAIN_2

5 - MAIN_2 / T1@MAIN_2

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Jonathan Lewis

© 2008 - 2009

Use of /*+ qb_name() */ (a)

select /*+ qb_name(main) */

{columns}

from t1, t3

where t1.n2 = 15

and exists (select /*+ qb_name(subq2) */ null

from t2

where t2.n1 = 15

and t2.id = t1.id

)

and t3.n1 = t1.n1

and t3.n2 = 15

and exists (select /*+ qb_name(subq4) */ null

from t4

where t4.n1 = 15

and t4.id = t3.id

)

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Jonathan Lewis

© 2008 - 2009

Use of /*+ qb_name() */ (b)

select

/*+

qb_name(main)

unnest(@subq2)

no_unnest(@subq4)

no_push_subq(@subq4)

leading(t1@main, t3@main, t2@subq2)

index(@subq4 t4@subq4(id))

*/

{columns}

from t1, t3

where t1.n2 = 15

and exists (select /*+ qb_name(subq2) */ ... t2 ...)

and t3.n1 = t1.n1

and t3.n2 = 15

and exists (select /*+ qb_name(subq4) */ ... t4 ...)

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Jonathan Lewis

© 2008 - 2009

Use of /*+ qb_name() */ (c)

.

| Id | Operation | Name | Rows |

| 0 | SELECT STATEMENT | | 1 |

| 1 | FILTER | | |

| 2 | NESTED LOOPS SEMI | | 3 |

| 3 | HASH JOIN | | 173 |

| 4 | TABLE ACCESS FULL | T1 | 157 |

| 5 | TABLE ACCESS FULL | T3 | 157 |

| 6 | TABLE ACCESS BY INDEX ROWID| T2 | 1 |

| 7 | INDEX UNIQUE SCAN | T2_PK | 1 |

| 8 | TABLE ACCESS BY INDEX ROWID | T4 | 1 |

| 9 | INDEX UNIQUE SCAN | T4_PK | 1 |

Query Block Name / Object Alias (identified by operation id):

1 - SEL$38F5B5F8

4 - SEL$38F5B5F8 / T1@MAIN

5 - SEL$38F5B5F8 / T3@MAIN

6 - SEL$38F5B5F8 / T2@SUBQ2

7 - SEL$38F5B5F8 / T2@SUBQ2

8 - SUBQ4 / T4@SUBQ4

9 - SUBQ4 / T4@SUBQ4

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Jonathan Lewis

© 2008 - 2009

Use of /*+ qb_name() */ (d)

/*+

BEGIN_OUTLINE_DATA

IGNORE_OPTIM_EMBEDDED_HINTS

OPTIMIZER_FEATURES_ENABLE('10.2.0.3')

OPT_PARAM('_optimizer_cost_model' 'io')

ALL_ROWS

OUTLINE(@"SUBQ4")

OUTLINE(@"MAIN")

OUTLINE(@"SUBQ2")

OUTLINE_LEAF(@"SUBQ4")

OUTLINE_LEAF(@"SEL$38F5B5F8")

UNNEST(@"SUBQ2")

FULL(@"SEL$38F5B5F8" "TA"@"MAIN")

FULL(@"SEL$38F5B5F8" "TC"@"MAIN")

INDEX_RS_ASC(@"SEL$38F5B5F8" "TB"@"SUBQ2" ("TB"."ID"))

LEADING(@"SEL$38F5B5F8" "TA"@"MAIN" "TC"@"MAIN" "TB"@"SUBQ2")

USE_HASH(@"SEL$38F5B5F8" "TC"@"MAIN")

USE_NL(@"SEL$38F5B5F8" "TB"@"SUBQ2")

INDEX_RS_ASC(@"SUBQ4" "TD"@"SUBQ4" ("TD"."ID"))

END_OUTLINE_DATA

*/

index_rs_asc() (and related hints) appeared in

10g specifically to deal with problems where

the index() hint resulted in index full scans.

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Jonathan Lewis

© 2008 - 2009

Use of /*+ qb_name() */ (e)

explain plan for ...

select * from table(dbms_xplan.display);

select *

from table(dbms_xplan.display(null,null,'{options}'));

select *

from table(

dbms_xplan.display(

null,

null,

'outline alias -cost -bytes'

)

)

;

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Jonathan Lewis

© 2008 - 2009

dbms_xplan (10gR2)

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

|Id |Operation |Name |Starts |E-Rows | A-Rows | A-Time | Buffers |

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

| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.07 | 1540 |

| 2 | VIEW | | 1 | 167 | 9998 |00:00:00.10 | 1540 |

|* 3 | FILTER | | 1 | | 9998 |00:00:00.07 | 1540 |

| 4 | TABLE ACCESS FULL |EMP | 1 | 20000 | 20000 |00:00:00.04 | 220 |

| 5 | SORT AGGREGATE | | 6 | 1 | 6 |00:00:00.03 | 1320 |

|* 6 | TABLE ACCESS FULL|EMP | 6 | 3333 | 20000 |00:00:00.04 | 1320 |

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

set serveroutput off

select /*+ gather_plan_statistics */ ...

select *

from table(dbms_xplan.display_cursor((null, null,'ALLSTATS LAST'));

You need the privileges to access:

v$session, v$sql, v$sql_plan_statistics(_all)

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Jonathan Lewis

© 2008 - 2009

What do hints mean (a)

select

/*+ ordered */

t1.v1

from t1, t3

where t3.n1 = t1.n1

and exists (

select t2.id

from t2

where t2.n1 = 15

and t2.id = t1.id

)

and exists (

select t4.id

from t4

where t4.n1 = 15

and t4.id = t3.id

);

But what is the one join order for this query ?

What is the impact of the subqueries ?

When does the hint operate ?

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

What do hints mean (b)

This was the 8i execution path

FILTER

HASH JOIN

TABLE ACCESS (FULL) OF T1


TABLE ACCESS (BY INDEX ROWID) OF T2

INDEX (RANGE SCAN) OF T2_N1 (NON-UNIQUE)

TABLE ACCESS (BY INDEX ROWID) OF T4

INDEX (RANGE SCAN) OF T4_N1 (NON-UNIQUE)

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Jonathan Lewis

© 2008 - 2009

What do hints mean (c)

This was the 9i execution path

NESTED LOOPS

NESTED LOOPS

MERGE JOIN (CARTESIAN)

SORT (UNIQUE)

TABLE ACCESS (BY INDEX ROWID) OF 'T4'

INDEX (RANGE SCAN) OF 'T4_N1' (NON-UNIQUE)

BUFFER (SORT)

SORT (UNIQUE)


INDEX (RANGE SCAN) OF 'T2_N1' (NON-UNIQUE)


INDEX (UNIQUE SCAN) OF 'T1_UK' (UNIQUE)


INDEX (UNIQUE SCAN) OF 'T3_UK' (UNIQUE)

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

What do hints mean (d)

select

/*+ ordered */

t1.v1

from (

select distinct t4.id

from t4

where t4.n1 = 15

) v4,

(

select distinct t2.id

from t2

where t2.n1 = 15

) v2,

t1, t3

where t3.n1 = t1.n1

and t2.id = t1.id

and t4.id = t3.id

;

A hint applies to a specific query block, and the optimizer transformed the

query into a single query block before (in this case) applying the hint.

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Jonathan Lewis

© 2008 - 2009

Precision matters (1)

select

/*+ noparallel(tabx) */

count(*)

from

TABX

;

This query may run parallel. Why ?

SELECT STATEMENT

SORT (AGGREGATE)

SORT* (AGGREGATE)

PARTITION RANGE* (ALL)

INDEX* (FULL SCAN) OF 'PT_PK' (UNIQUE)

You didn’t say: noparallel_index(TABX)

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Jonathan Lewis

© 2008 - 2009

Precision matters (2a)

select /*+ parallel(t1,2) */

{list of columns}

from t1

where t1.n1 = 5

and t1.n2 = 10

and t1.pt_group in (0,10)

;

t1 is list-partitioned on pt_group,

with a local index on (n1, n2).

The query visits two partitions

.

|Id|Operation |Name |Cost |Pstart| Pstop|

| 0|SELECT STATEMENT | | 48| | |

| 1| PARTITION LIST INLIST | | 48|KEY(I)|KEY(I)|

|*2| TABLE ACCESS BY LOCAL INDEX ROWID|T1 | 48|KEY(I)|KEY(I)|

|*3| INDEX RANGE SCAN |T1_I1| 3|KEY(I)|KEY(I)|

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Jonathan Lewis

© 2008 - 2009

Precision matters (2b)

select /*+ full(t1) parallel(t1,2) */

{list of columns}

from t1

where t1.n1 = 5

and t1.n2 = 10


;.

| Id| Operation |Name |Cost |Pstart|Pstop | TQ |IN-OUT|PQ Distrib|

| 0| SELECT STATEMENT | | 94 | | | | | |

| 1| PX COORDINATOR | | | | | | | |

| 2| PX SEND QC (RANDOM)|:TQ10| 94 | | |Q1,00| P->S |QC (RAND) |

| 3| PX BLOCK ITERATOR | | 94 |KEY(I)|KEY(I)|Q1,00| PCWC | |

|* 4| TABLE ACCESS FULL|T1 | 94 |KEY(I)|KEY(I)|Q1,00| PCWP | |

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Jonathan Lewis

© 2008 - 2009

Precision matters (2c)

select /*+ parallel(t1,4) */

{list of columns}

from t1

where t1.n1 = 5

and t1.n2 = 10


;.

| Id| Operation |Name |Cost |Pstart|Pstop | TQ |IN-OUT|PQ Distrib|

| 0| SELECT STATEMENT | | 47 | | | | | |

| 1| PX COORDINATOR | | | | | | | |

| 2| PX SEND QC (RANDOM)|:TQ10| 47 | | |Q1,00| P->S |QC (RAND) |

| 3| PX BLOCK ITERATOR | | 47 |KEY(I)|KEY(I)|Q1,00| PCWC | |

|* 4| TABLE ACCESS FULL|T1 | 47 |KEY(I)|KEY(I)|Q1,00| PCWP | |

For indexed paths, the number of

slaves used will be a maximum of one

per partition – but for tablescans this

limit can be exceeded.

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Jonathan Lewis

© 2008 - 2009

Precision matters (3a)

How can Oracle do this:

HASH JOIN



select /*+ use_nl(t2) */

t1.col1, t2.col2

from t1, t2

where t2.pkcol = t1.pkcol

and t2.spare_col = 99;

The tables seem to be in the

expected join order, using

the wrong join mechanism.

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

Precision matters (3b)

Join order[1]: T1 [T1] T2 [T2]

Now joining: T2[T2] *******

NL join



NL join

SM Join

HA Join -- (sides swapped)

From the 10053 trace file:

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Jonathan Lewis

© 2008 - 2009

Precision matters (3c)



NL join



NL join

It’s a short-hand

for:

/*+

use_nl(t1)

use_nl(t2)

*/

select /*+ use_nl(t1 t2) */

t1.col1, t2.col2

from t1, t2



Why might this appear to work ?

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Jonathan Lewis

© 2008 - 2009

Precision matters (3d)

select /*+

ordered -- table order

use_nl(t2) -- join method

full(t1) -- access method

index(t2(pkcol)) -- access method

*/

t1.col1,

t2.col2

from t1, t2



The correct hints

Note the 10g style of index hint –

which lists, in order, the starting

columns of a suitable index.

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Jonathan Lewis

© 2008 - 2009

Precision matters (4)

...

NESTED LOOP


TABLE ACCESS BY INDEX ROWID T2

INDEX RANGE SCAN T2_IND_SECOND -- “wrong” index

...

select /*+ index(t2 t2_ind_first) */ -- hint the “right” one

...

HASH JOIN -- wrong method


TABLE ACCESS BY INDEX ROWID T2

INDEX FULL SCAN T2_IND_FIRST -- “right” index

...

select /*+ index_rs_asc(t2 t2_ind_first) */ -- the right hint

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

Plans and join order (1)

select

/*+

ordered full(t4)

use_hash(t1) full(t1)



*/

count(t1.colx), count(t2.coly), ...

from

t4, t1, t2, t3

where

t1.id = t4.id1

and t2.id = t4.id2

and t3.id = t4.id3

;

How many different (serial) executions

plans are there that obey this set of hints ?

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Jonathan Lewis

© 2008 - 2009


N – 1 simultaneous hash tables

Maximum two simultaneous hash tables

HASH JOIN

TABLE ACCESS (FULL) OF TABLE_3

HASH JOIN


HASH JOIN



HASH JOIN

HASH JOIN

HASH JOIN





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Jonathan Lewis

© 2008 - 2009


select

/*+

ordered full(t4)

use_hash(t1) full(t1) swap_join_inputs(t1)



*/

count(t1.colx), count(t2.coly), ...

from

t4, t1, t2, t3

where

t1.id = t4.id1

and t2.id = t4.id2

and t3.id = t4.id3

;

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Jonathan Lewis

© 2008 - 2009


This join order really was: t4, t1, t2, t3

HASH JOIN


HASH JOIN


HASH JOIN



A hash join is always allowed to swap the order of its join inputs.

It takes two hints to specify a hash join completely. 9i gave us

/*+ swap_join_inputs(second_table)*/

And 10g finished the job with:

/*+ no_swap_join_inputs(second_table)*/

Hints on Hints

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Jonathan Lewis

© 2008 - 2009


Join order in from clause: T4 T1 T2 T3

Now joining T1: (T4, T1)

swap_join_inputs(t1) (T1, T4)

Now joining T2: ((T1, T4), T2)

swap_join_inputs(t2) (T2, (T1, T4))

Now joining T3: ((T2, (T1, T4)), T3)

swap_join_inputs(t3) (T3, (T2, (T1, T4)))

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Jonathan Lewis

© 2008 - 2009

The story so far

• “Simple” hints aren’t so simple

• You need lots more hints than you think

• If you manage to get the plan you want –

– Use dbms_xplan(null,null,’outline’) to get the full set of

hints, and use the entire list into your SQL.

• And put back any qb_name and parallelism hints

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

Experimenting (a)

select /*+

parallel(t1 3) parallel(t2 3)

full(t1) full(t2)

use_hash(t2) pq_distribute(t2 hash hash)

*/

t1.padding, t2.padding

from t1, t2

where t2.n1 = t1.n1

and t2.small_vc = t1.small_vc

Execution plan 9.2.0.8 .

| Id| Operation | Name| Rows | Bytes | Cost | TQ |IN-OUT| PQ Distrib |

| 0| SELECT STATEMENT | | 2 | 1444 | 164 | | | |

|* 1| HASH JOIN | | 2 | 1444 | 164 | 98,02 | P->S | QC (RAND) |

| 2| TABLE ACCESS FULL| T1 |10325 | 2127K| 24 | 98,00 | P->P | HASH |

| 3| TABLE ACCESS FULL| T2 |20212 | 9M| 118 | 98,01 | P->P | HASH |

This spilled to disk – “unnecessarily”.

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Jonathan Lewis

© 2008 - 2009

Experimenting (b)

Q002

Hash join

Q000, Q001

Scan para_1, para_2

Co-ordinator

Slave P001 Slave P002Slave P000


Total number of rows in in-memory partitions: 5029

(used as preliminary number of buckets in hash table)

Estimated max # of build rows that can fit in avail memory: 78676

### Partition Distribution ###

Partition:0 rows:618 clusters:1 slots:1 kept=1

. . .

Partition:7 rows:610 clusters:1 slots:1 kept=1

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Jonathan Lewis

© 2008 - 2009

Experimenting (c)


|Id|Operation | Name | Rows |Bytes | Cost | TQ |IN-OUT| PQ Distrib |

| 0|SELECT STATEMENT | | 2| 1460 | 181 | | | |

| 1| PX COORDINATOR | | | | | | | |

| 2| PX SEND QC (RANDOM) | :TQ10002| 2| 1460 | 181 | Q1,02 | P->S | QC (RAND) |

|*3| HASH JOIN BUFFERED | | 2| 1460 | 181 | Q1,02 | PCWP | |

| 4| PX RECEIVE | | 10000| 2099K| 28 | Q1,02 | PCWP | |

| 5| PX SEND HASH | :TQ10000| 10000| 2099K| 28 | Q1,00 | P->P | HASH |

| 6| PX BLOCK ITERATOR | | 10000| 2099K| 28 | Q1,00 | PCWC | |

| 7| TABLE ACCESS FULL| T1 | 10000| 2099K| 28 | Q1,00 | PCWP | |

| 8| PX RECEIVE | | 20000| 9M| 131 | Q1,02 | PCWP | |

| 9| PX SEND HASH | :TQ10001| 20000| 9M| 131 | Q1,01 | P->P | HASH |

|10| PX BLOCK ITERATOR | | 20000| 9M| 131 | Q1,01 | PCWC | |

|11| TABLE ACCESS FULL| T2 | 20000| 9M| 131 | Q1,01 | PCWP | |

The complete result set for the join is “buffered” before being forwarded to the query co-ordinator.

Even though the hash table fits in memory, the result set does not – so it spilled to temp and was re-read.

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Jonathan Lewis

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Experimenting (d)


|Id|Operation | Name | Rows |Bytes | Cost | TQ |IN-OUT| PQ Distrib |

| 0|SELECT STATEMENT | | 2| 1460 | 181 | | | |

| 1| PX COORDINATOR | | | | | | | |

| 2| PX SEND QC (RANDOM) | :TQ10002| 2| 1460 | 181 | Q1,02 | P->S | QC (RAND) |

|*3| HASH JOIN | | 2| 1460 | 181 | Q1,02 | PCWP | |

| 4| PX RECEIVE | | 10000| 2099K| 28 | Q1,02 | PCWP | |

| 5| PX SEND HASH | :TQ10000| 10000| 2099K| 28 | Q1,00 | P->P | HASH |

| 6| PX BLOCK ITERATOR | | 10000| 2099K| 28 | Q1,00 | PCWC | |

| 7| TABLE ACCESS FULL| T1 | 10000| 2099K| 28 | Q1,00 | PCWP | |

| 8| PX RECEIVE | | 20000| 9M| 131 | Q1,02 | PCWP | |

| 9| PX SEND HASH | :TQ10001| 20000| 9M| 131 | Q1,01 | P->P | HASH |

|10| PX BLOCK ITERATOR | | 20000| 9M| 131 | Q1,01 | PCWC | |

|11| TABLE ACCESS FULL| T2 | 20000| 9M| 131 | Q1,01 | PCWP | |

The hint list (in 11g) showed: /*+ no_qkn_buff */ - “No Query Ko-ordiNator BUFFering” ?

The QC jammed on “PX Deq: Execute reply” – 2 second timeout

Lower slaves jammed on “PX Deq: Execution Msg” – 1/10 second timeout

Upper slaves jammed on “PX Deq: Send blkd” – 2 second timeout

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

Experimenting (e)

/*+ pq_distribute(t2 broadcast none) */


| Id| Operation | Name| Rows | Bytes | Cost | TQ |IN-OUT| PQ Distrib |

| 0| SELECT STATEMENT | | 2 | 1444 | 163 | | | |

|* 1| HASH JOIN | | 2 | 1444 | 163 | 16,01 | P->S | QC (RAND) |

| 2| TABLE ACCESS FULL| T1 |10000 | 2060K| 24 | 16,00 | P->P | BROADCAST |

| 3| TABLE ACCESS FULL| T2 |20000 | 9980K| 118 | 16,01 | PWCP | |

Q001

Scan para_2 & join

Q000

Scan para_1

Co-ordinator



23

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

“Feature” Hints

More common in 11g

Switch off new features (matched to parameters)

Probably safe to use

For example:

cursor_sharing_exact bypasses cursor_sharing = ...

no_set_to_join disable set to join conversion

no_use_hash_aggregation disable hash aggregation

no_eliminate_join disable join elimination

no_eliminate_oby disable “order by” elimination

opt_param('parameter','value')

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

“Strategic” Hints

Shape the query, without forcing details:

unnest / no_unnest whether to transform subqueries

push_subq / no_push_subq when to run subqueries

merge / no_merge control handling of complex views

push_pred / no_push_pred how to handle non-mergeable views

driving_site() where to run a distributed query

Basically these are hints about query blocks

24

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Jonathan Lewis

© 2008 - 2009

Unnesting (a)

select

outer.*

from

emp outer

where

outer.sal > (

select

/*+ unnest redundant for 9i */

avg(inner.sal)

from emp inner

where inner.dept_no = outer.dept_no

)

;

Hints on Hints

48 / 78

Jonathan Lewis

© 2008 - 2009

Unnesting (b)

select dept_no, avg(sal) av_sal

from emp

group by dept_no

select -- notional transformation (9i)

outer.*

from

(

select dept_no, avg(sal) av_sal

from emp

group by dept_no

) inner,

emp outer

where

outer.dept_no = inner.dept_no

and outer.sal > inner.av_sal;

25

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Jonathan Lewis

© 2008 - 2009

Unnesting (c)

select -- investigated transformation (10g)

outer.dept_no dept_no, outer.sal sal,

outer.emp_no emp_no, outer.padding padding

from

test_user.emp inner, test_user.emp outer

where

inner.dept_no = outer.dept_no

group by

inner.dept_no, outer.rowid,

outer.padding, outer.emp_no,

outer.sal, outer.dept_no

having

outer.sal > avg(inner.sal)

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

Subquery caching (a)

select

/*+ qb_name(main) */

outer.*

from

emp outer

where

outer.sal > (

select

/*+ no_unnest qb_name(subq)*/

avg(inner.sal)

from emp inner


)

;

26

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

Subquery caching (b)

select

outer.*

from (

select /*+ no_merge */

emp1.*

from emp emp1

order by

emp1.dept_no

) outer

where

outer.sal > (

select /*+ no_unnest */

avg(inner.sal)

from emp inner


)

;

select

emp1.*

from emp emp1

order by

emp1.dept_no

Hints on Hints

52 / 78

Jonathan Lewis

© 2008 - 2009

Subquery caching (c)

select /*+ no_merge(@ordered) no_eliminate_oby(@ordered) */

outer.*

from (

select /*+ qb_name(ordered) */

emp1.*

from emp emp1

order by

emp1.dept_no

) outer

where

outer.sal > (

select /*+ no_unnest */

avg(inner.sal)

from emp inner


)

;

27

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53 / 78

Jonathan Lewis

© 2008 - 2009

Subquery pushdown (1)

select

/*+ ordered */

t1.v1

from t1, t3

where t1.n2 = 15

/*

and exists (select --+ no_unnest qb_name(subq2)

null

from t2

where t2.n1 = 15

and t2.id = t1.id

)

*/

and t3.n1 = t1.n1

and t3.n2 = 15

;

Hints on Hints

54 / 78

Jonathan Lewis

© 2008 - 2009


Execution Plan 9.2.0.8 – without subquery .



|* 1 | HASH JOIN | | 173 | 3979 | 109 |

|* 2 | TABLE ACCESS FULL | T1 | 157 | 2355 | 54 |


Execution Plan 9.2.0.8 – with subquery .



|* 1 | FILTER | | | | |

|* 2 | HASH JOIN | | 9 | 252 | 109 |



|* 5 | TABLE ACCESS BY INDEX ROWID| T2 | 1 | 8 | 2 |

|* 6 | INDEX UNIQUE SCAN | T2_PK | 1 | | 1 |

28

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Jonathan Lewis

© 2008 - 2009


select

/*+ ordered push_subq */ -- 9i use of hint

/*+ ordered push_subq(@subq2) */ -- 10g use of hint

/*+ no_unnest(@subq2) */ -- 10g use of hint

t1.v1

From t1, t3

Where t1.n2 = 15

and exists (select --+ no_unnest qb_name(subq2) push_subq

null

from t2

where t2.n1 = 15

and t2.id = t1.id

)

and t3.n1 = t1.n1

and t3.n2 = 15

;

Alternative 10g use of hint.

Hints on Hints

56 / 78

Jonathan Lewis

© 2008 - 2009


Execution Plan 10.2.0.3 – without pushing .

| Id | Operation | Name | Rows | Bytes | Cost(%CPU)| Time |

| 0 | SELECT STATEMENT | | 1 | 28 | 369 (2)| 00:00:05 |

|* 1 | FILTER | | | | | |

|* 2 | HASH JOIN | | 173 | 4844 | 195 (2)| 00:00:03 |

|* 3 | TABLE ACCESS FULL | T1 | 157 | 3140 | 97 (2)| 00:00:02 |


|* 5 | TABLE ACCESS BY INDEX ROWID| T2 | 1 | 8 | 2 (0)| 00:00:01 |

|* 6 | INDEX UNIQUE SCAN | T2_PK | 1 | | 1 (0)| 00:00:01 |

Execution Plan 10.2.0.3 – pushed .

| Id | Operation | Name | Rows | Bytes | Cost(%CPU)| Time |

| 0 | SELECT STATEMENT | | 9 | 252 | 197 (2)| 00:00:03 |

|* 1 | HASH JOIN | | 9 | 252 | 195 (2)| 00:00:03 |

|* 2 | TABLE ACCESS FULL (filter) | T1 | 8 | 160 | 97 (2)| 00:00:02 |

|* 3 | TABLE ACCESS BY INDEX ROWID| T2 | 1 | 8 | 2 (0)| 00:00:01 |

|* 4 | INDEX UNIQUE SCAN | T2_PK | 1 | | 1 (0)| 00:00:01 |


29

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Jonathan Lewis

© 2008 - 2009

Non-mergeable views (a)

Customers

Orders

Suppliers

Products

Order_lines

List all orders supplied to

customers in London that

contain products from

suppliers in Leeds

There may not be many of

them, but there is no

efficient way to find them.

The only filters are at the

ends of a five table join

Hints on Hints

58 / 78

Jonathan Lewis

© 2008 - 2009

Non-mergeable views (b)

Customers

Orders

Suppliers

Products

Order_lines

select /*+ ordered

use_hash(prdsup) swap_join_inputs(prdsup)

use_hash(cusord) swap_join_inputs(cusord)

*/ cusord.*

from

order_lines orl,

(

select /*+ no_merge */ prd.id

from products prd, suppliers sup

where sup.location = 'Leeds'

and prd.supp_id = sup.id

) prdsup,

(

select /*+ no_merge */ ord.*

from customers cus, orders ord

where cus.location = 'London'

and ord.cust_id = cus.id

) cusord

where orl.order_id = cusord.id

and prdsup.id = orl.product_id;

30

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Jonathan Lewis

© 2008 - 2009

Non-mergeable views (c)

1 HASH JOIN

2 VIEW -- (1) hash this view

3 HASH JOIN

4 TABLE ACCESS (FULL) OF 'CUSTOMERS'

5 TABLE ACCESS (FULL) OF 'ORDERS'

6 HASH JOIN

7 VIEW -- (2) hash this view

8 HASH JOIN

9 TABLE ACCESS (FULL) OF 'SUPPLIERS'

10 TABLE ACCESS (FULL) OF 'PRODUCTS'

11 TABLE ACCESS (FULL) OF 'ORDER_LINES‘ -- (3) scan this table

Order lines are passed through the products/suppliers hash, then

through the customers/orders hash. This could use a lot of memory.

Hints on Hints

60 / 78

Jonathan Lewis

© 2008 - 2009

Bushy tree

31

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

Left-deep tree

Hints on Hints

62 / 78

Jonathan Lewis

© 2008 - 2009

Pushing Join Predicates (a)

create or replace view v1 as

select

t2.id1, t2.id2, t3.small_vc, t3.padding

from t2, t3

where t3.id1 = t2.id1

and t3.id2 = t2.id2

;

select /*+ push_pred(v1) */

t1.*, v1.*

from t1, v1

where v1.id1(+) = t1.id1

and t1.n1 = 5

;

32

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Jonathan Lewis

© 2008 - 2009

Pushing Join Predicates (b)

Plan before predicate pushing .| Id | Operation | Name | Rows | Bytes | Cost |


|* 1 | HASH JOIN OUTER | | 50 | 10200 | 55 |


| 3 | VIEW | V1 | 25000 | 2075K| 44 |

| 4 | NESTED LOOPS | | 25000 | 3149K| 44 |

| 5 | TABLE ACCESS FULL| T3 | 25000 | 2929K| 44 |

|* 6 | INDEX UNIQUE SCAN| T2_PK | 1 | 9 | |


1 - access("V1"."ID1"(+)="T1"."ID1")

2 - filter("T1"."N1"=5)

6 - access("T3"."ID1"="T2"."ID1" AND "T3"."ID2"="T2"."ID2")

Hints on Hints

64 / 78

Jonathan Lewis

© 2008 - 2009

Pushing Join Predicates (c)

Plan with predicate pushing .| Id | Operation | Name | Rows | Bytes | Cost |


| 1 | NESTED LOOPS OUTER | | 50 | 10300 | 40 |


| 3 | VIEW PUSHED PREDICATE | V1 | 1 | 87 | 3 |

| 4 | NESTED LOOPS | | 1 | 133 | 3 |


|* 6 | INDEX RANGE SCAN | T3_PK | 5 | | 2 |

|* 7 | INDEX UNIQUE SCAN | T2_PK | 1 | 13 | |


2 - filter("T1"."N1"=5)

6 - access("T3"."ID1"="T1"."ID1")

7 - access("T2"."ID1"="T1"."ID1" AND "T3"."ID2"="T2"."ID2")

filter("T3"."ID1"="T2"."ID1")

33

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Jonathan Lewis

© 2008 - 2009

Distributed Effects

create table dist_home {dist_away} as

select

rownum id,

rpad(rownum,10) small_vc,

rpad(rownum,200) large_vc

from all_objects

where rownum <= 2000;

alter table dist_home {dist_away}

add constraint dh_pk {da_pk} primary key (id);

create public database link d920@loopback using 'd920';

Hints on Hints

66 / 78

Jonathan Lewis

© 2008 - 2009

Distributed Effects

select

/*+ driving_site (dh) */

dh.small_vc,

da.large_vc

from

dist_home dh,

dist_away@d920@loopback da

where

dh.small_vc like '12%'

and da.id = dh.id

;

34

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Jonathan Lewis

© 2008 - 2009

Distributed Effects

ID Row Source

0 SELECT STATEMENT (all_rows)

1 NESTED LOOPS

2 TABLE ACCESS(analyzed)DIST_HOME full

3 REMOTE SERIAL_FROM_REMOTE

3 D920@LOOPBACK -- object_node

3 SELECT "ID","LARGE_VC" -- other

FROM "DIST_AWAY" "DA"

WHERE "ID"=:1 -- read consistency ?

Hints on Hints

68 / 78

Jonathan Lewis

© 2008 - 2009

Distributed Effects

Change the filter predicate to select more data:

dh.small_vc like '1%'

ID Row Source

0 SELECT STATEMENT (all_rows)

1 HASH JOIN

2 TABLE ACCESS(analyzed)DIST_HOME full


3 D920@LOOPBACK

3 SELECT "ID","LARGE_VC"

FROM "DIST_AWAY" "DA"

35

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Jonathan Lewis

© 2008 - 2009

Distributed Effects

ID Row Source

0 SELECT STATEMENT (all_rows)(remote)

1 HASH JOIN


3 TABLE ACCESS(analyzed)DIST_AWAY full

2 ! -- where is this node ?

3 D920@LOOPBACK -- this one knows who it is

“Tune” the hash join by changing the driving site to the away d/b

/*+ driving_site (da) */

2 SELECT "ID","SMALL_VC"

FROM "DIST_HOME" "A2"

WHERE "SMALL_VC" LIKE '1%'

Hints on Hints

70 / 78

Jonathan Lewis

© 2008 - 2009

Multi-table distributed joins

select ... from

sales@d920@loopback sal,

sites sit,

products@d920@loopback prd

where ...

SELECT STATEMENT Optimizer=ALL_ROWS

HASH JOIN

INDEX (FULL SCAN) OF 'SI_PK' (UNIQUE)

REMOTE* SERIAL_FROM_REMOTE D920.JLCOMP.CO.UK@LOOPBACK

SELECT

"A1"."SALE_DATE", ... "A2"."PROMOTED"

FROM

"SALES" "A1","PRODUCTS" "A2"

WHERE ...

36

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Jonathan Lewis

© 2008 - 2009

Multi-table distributed joins

0 SELECT STATEMENT Optimizer=ALL_ROWS

1 NESTED LOOPS

2 NESTED LOOPS

3 REMOTE* D920.JLCOMP.CO.UK@LOOPBACK

4 INDEX (UNIQUE SCAN) OF 'SI_PK' (UNIQUE)

5 REMOTE* D920.JLCOMP.CO.UK@LOOPBACK

3 SELECT "SALE_DATE","SITE","PRODUCT","QTY","PROFIT"

FROM "SALES" "SAL"

5 SELECT /*+ INDEX("PRD") USE_NL("PRD") */ "ID","PROMOTED"

FROM "PRODUCTS" "PRD"

WHERE "ID"= :1

AND "PROMOTED">TO_DATE('2005-04-19 00:00:00', 'yyyy-mm-dd hh24:mi:ss')

Hints on Hints

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Jonathan Lewis

© 2008 - 2009

Controlling distributed (a)

select bv.*

from ext_view bv -- remote view

where

bv.objectid in (

select linkid

from links_table -- local table

where projectid = 116448

and blocklist = 'N'

)

and bv.gisdatasetname = 'XXX'

order by

bv.physical_length desc

37

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Jonathan Lewis

© 2008 - 2009

Controlling distributed (b)

SORT ORDER BY

NESTED LOOPS SEMI

REMOTE EXT_VIEW

TABLE ACCESS BY INDEX ROWID LINKS_TABLE

INDEX UNIQUE SCAN PK_PGM

When I run the (local) subquery by itself it returns a very small number of rows

When I run the main (remote) query with an in-list of

those values it executes very quickly

But I can’t make Oracle do this automatically

Hints on Hints

74 / 78

Jonathan Lewis

© 2008 - 2009

Controlling distributed (c)

select bv.*

from (

select distinct link_id

from links_table

where projectid = 116448

and blocklist = 'N'

) lk,

ext_view bv

where bv.object_id = lk.linkid

and bv.gisdatasetname = 'XXX'

order by


;

38

Hints on Hints

75 / 78

Jonathan Lewis

© 2008 - 2009

Controlling distributed (d)

select

/*

ordered no_merge(lk) no_merge(bv)

use_nl(bv) push_pred(bv)

*/

bv.*

from (

select distinct link_id

from links_table

where projectid = 116448 and blocklist = 'N'

) lk,

my_view bv

where bv.object_id = lk.linkid and bv.gisdatasetname = 'XXX'

order by


;

Hints on Hints

76 / 78

Jonathan Lewis

© 2008 - 2009

How to use hints (1)

• Set the join order– Ordered, leading (t1, t2, …) (10g version)

– But remember unnesting problems with ordered

• Join method for N-1 tables– Use_nl, use_merge, use_hash + (no_)swap_join_inputs

• Access path for every table– Index, no_index, hash, full, index_ffs etc.

• Average two hints per table to do it well– (three if you use hash joins)

39

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Jonathan Lewis

© 2008 - 2009

How to use hints (2)

• Block or force unnesting– unnest, no_unnest - in the subquery

• Block or force early subqueries– push_subq, no_push_subq – change syntax in 10g

• Block or force merging– merge, no_merge - two forms, with or w/o alias

• Block, or force, predicate pushing– push_pred, no_push_pred (10g)

• Choosing the driving site for distributed– Driving_site()

Hints on Hints

78 / 78

Jonathan Lewis

© 2008 - 2009

Summary

Avoid ‘micro-management’ hints

“Strategic” hints can be useful and safe

“Feature” hints can be useful and safe

Keep testing on every upgrade

Use query block names from 10g onwards

hints on hints - oracle scratchpad | just another oracle · 4 hints on hints 7 / 78 jonathan...

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