Category Archives: oracle

Thoughts on Big Data SQL SmartScan

Posted on by Posted in cell_offload, External tables, oracle, SmartScan 1,653 Page views Leave a comment

Just a few thoughts I’ve been meaning to blog for a while:

1. Number of columns that can be offloaded

Exadata

Very early on in the Exadata Smart Scan implementation a restriction was added to prevent offloading more than 255 columns. This was done because of performance issues observed by customers. Smart Scan works on 1 MB chunks at a time and needs all the row pieces up to the highest #segcol referenced to be present in the 1 MB chunk that is being processed. When rows with more than 255 columns are first inserted all the row pieces are contiguous and hence likely to be present in the 1 MB chunk.

However, if the DBA has not provided sufficient freespace (PCTFREE) in the block when it is first loaded subsequent DMLs may end up splitting row pieces and moving pieces to blocks in the freelist that have more space available. PCTFREE specifies how much empty space to leave for future updates when a block is first populated.

Continue reading→

Reading and analyzing trace file contents using just SQL

Posted on by Posted in 12.2, oracle, trace, troubleshooting 1,688 Page views Leave a comment

Simple example: tracefiles for the last 5 days:

select fc.* 
from v$diag_trace_file f
     join v$diag_trace_file_contents fc
          on f.adr_home=fc.adr_home
          and f.trace_filename=fc.trace_filename
where f.modify_time >= systimestamp - interval'5' minute
  and fc.timestamp  >= systimestamp - interval'5' minute
  and fc.component_name = 'SQL_Trace'
  --and fc.section_name like 'kests%'
  ;
--or:
select tr.*
  from v$diag_app_trace_file tf,
       v$diag_sql_trace_records tr
 where tf.sql_trace = 'Y'
   and tf.modify_time > systimestamp - interval'5'minute
   and tr.adr_home = tf.adr_home
   and tr.trace_filename = tf.trace_filename
   and tr.timestamp > systimestamp - interval'5'minute;

Laterals: is (+) documented for laterals?

Posted on by Posted in bug, CBO, documentation, oracle 1,553 Page views Leave a comment

I know this syntax for a long time, since when lateral() was not documented yet, but recently I found a bug: the following query successfully returns 1 row:

with a as (select level a from dual connect by level<10)
    ,b as (select 0 b from dual)
    ,c as (select 0 c from dual)
select
  *
from a,
     lateral(select * from b where a.a=b.b)(+) bb
     --left outer join c on c.c=bb.b
where a=1;

         A          B
---------- ----------
         1

But doesn’t if we uncomment “left join”:

with a as (select level a from dual connect by level<10)
    ,b as (select 0 b from dual)
    ,c as (select 0 c from dual)
select
  *
from a,
     lateral(select * from b where a.a=b.b)(+) bb
     left outer join c on c.c=bb.b
where a=1;

no rows selected

And outer apply works fine:

with a as (select level a from dual connect by level<10)
    ,b as (select 0 b from dual)
    ,c as (select 0 c from dual)
select
  *
from a
     outer apply (select * from b where a.a=b.b) bb
     left outer join c on c.c=bb.b
where a=1;

         A          B          C
---------- ---------- ----------
         1

flashback_transaction_query: fixed table full table scan

Posted on by Posted in hints, oracle, query optimizing 2,007 Page views Leave a comment

Recently I got a question: why CBO chooses “FIXED TABLE FULL TABLE” scan instead of “FIXED INDEX” in case of a join to flashback_transaction_query view with the predicate “q.xid = a.versions_xid”:

select
  versions_xid,
  versions_startscn,
  versions_starttime,
  versions_endscn,
  versions_endtime,
  --scn_to_timestamp,
  to_char(a.classification), a.* 
from vms.advisory_arch_20190624 versions between
         timestamp timestamp'2019-06-24 13:28:00' and maxvalue a
    join flashback_transaction_query q
      on q.xid = a.versions_xid
 where a.advisoryid = 100511;

As you can see flashback_transaction_query is just a wrapper view for SYS.X$KTUQQRY, ie fixed table, which obviously has good “fixed index” for that:

And simple query with the predicate xid=hextoraw(…) uses it:

In such cases we need to check another part of the equality predicate, because usually it means that we use some function instead of literal, bind variable or SQL operator. Simple example with v$session_event:

explain plan for
SELECT SUM(E.TIME_WAITED)/100 AS TIME_WAITED 
FROM V$SESSION_EVENT E 
WHERE E.SID = SYS_CONTEXT('userenv', 'sid')
AND (E.EVENT = 'TCP Socket (KGAS)' OR E.EVENT LIKE 'SQL*Net%dblink');

But it works fine with bind variables or if we use “precompute_subquery” hint to replace function call with the computed literal:

SQL> explain plan for
  2  SELECT  SUM(E.TIME_WAITED)/100 AS TIME_WAITED
  3  FROM  V$SESSION_EVENT E
  4  WHERE E.SID = :sid
  5    AND (E.EVENT = 'TCP Socket (KGAS)' OR E.EVENT LIKE 'SQL*Net%dblink');

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
Plan hash value: 404853953

---------------------------------------------------------------------------------------------
| Id  | Operation                 | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT          |                 |     1 |    47 |     1 (100)| 00:00:01 |
|   1 |  SORT AGGREGATE           |                 |     1 |    47 |            |          |
|   2 |   NESTED LOOPS            |                 |     3 |   141 |     1 (100)| 00:00:01 |
|*  3 |    FIXED TABLE FIXED INDEX| X$KSLES (ind:1) |   403 |  6851 |     1 (100)| 00:00:01 |
|*  4 |    FIXED TABLE FIXED INDEX| X$KSLED (ind:2) |     1 |    30 |     0   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter("S"."KSLESSID"=TO_NUMBER(:SID) AND "S"."KSLESWTS"<>0 AND
              "S"."INST_ID"=USERENV('INSTANCE'))
   4 - filter(("D"."KSLEDNAM"='TCP Socket (KGAS)' OR "D"."KSLEDNAM" LIKE
              'SQL*Net%dblink') AND "S"."KSLESENM"="D"."INDX")
---------------------------------------------------------------------------------
---------------------------------------------------------------------------------
  1  explain plan for
  2  SELECT SUM(E.TIME_WAITED)/100 AS TIME_WAITED
  3  FROM
  4   V$SESSION_EVENT E
  5  WHERE E.SID in (select/*+ precompute_subquery */ SYS_CONTEXT('userenv', 'sid') from dual) AND (E.EVENT =
  6*   'TCP Socket (KGAS)' OR E.EVENT LIKE 'SQL*Net%dblink')
SQL> /

Explained.

SQL> @xplan ""



PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
Plan hash value: 2330447422

---------------------------------------------------------------------------------------------
| Id  | Operation                 | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT          |                 |     1 |    50 |     0   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE           |                 |     1 |    50 |            |          |
|   2 |   NESTED LOOPS            |                 |     3 |   150 |     0   (0)| 00:00:01 |
|*  3 |    FIXED TABLE FIXED INDEX| X$KSLES (ind:1) |   514 | 10280 |     0   (0)| 00:00:01 |
|*  4 |    FIXED TABLE FIXED INDEX| X$KSLED (ind:2) |     1 |    30 |     0   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter("S"."KSLESSID"=382 AND "S"."KSLESWTS"<>0 AND ("S"."CON_ID"=0 OR
              "S"."CON_ID"=3) AND "S"."INST_ID"=USERENV('INSTANCE'))
   4 - filter("S"."KSLESENM"="D"."INDX" AND ("D"."KSLEDNAM"='TCP Socket (KGAS)' OR
              "D"."KSLEDNAM" LIKE 'SQL*Net%dblink'))

19 rows selected.

So the same problem we got with flashback_transaction_query, because VERSIONS_XID is a pseudocolumn in fact (ie function). So we can easily hide it in inline view: 

SQL> ;
  1  explain plan for
  2  with a as (
  3  select/*+ no_merge */
  4         versions_xid, versions_startscn, versions_starttime, versions_endscn, versions_endtime, --scn_to_timestamp,
  5         to_char(x.classification),
  6         x.*
  7    from advisory_arch_20190624 versions between timestamp to_timestamp('2019-06-25 22:14:00', 'YYYY-MM-DD HH24:MI:SS') and maxvalue x
  8    where x.advisoryid = 100511
  9  )
 10  select/*+ leading(a q) use_nl(q) index(q (xid)) */ *
 11  from a
 12*      join flashback_transaction_query q on q.xid = a.versions_xid
SQL> /

Explained.

SQL> @xplan +outline

P_FORMAT
------------------------
typical +outline

1 row selected.


PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------
Plan hash value: 2200855354

---------------------------------------------------------------------------------------------------
| Id  | Operation                | Name                   | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |                        |     1 |  2596 |     6   (0)| 00:00:01 |
|   1 |  NESTED LOOPS            |                        |     1 |  2596 |     6   (0)| 00:00:01 |
|   2 |   VIEW                   |                        |     1 |    98 |     6   (0)| 00:00:01 |
|*  3 |    TABLE ACCESS FULL     | ADVISORY_ARCH_20190624 |     1 |     9 |     6   (0)| 00:00:01 |
|*  4 |   FIXED TABLE FIXED INDEX| X$KTUQQRY (ind:1)      |     1 |  2498 |     0   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------------

Outline Data
-------------

  /*+
      BEGIN_OUTLINE_DATA
      FULL(@"SEL$4" "X"@"SEL$4")
      USE_NL(@"SEL$62A6D27E" "X$KTUQQRY"@"SEL$3")
      LEADING(@"SEL$62A6D27E" "A"@"SEL$1" "X$KTUQQRY"@"SEL$3")
      FULL(@"SEL$62A6D27E" "X$KTUQQRY"@"SEL$3")
      NO_ACCESS(@"SEL$62A6D27E" "A"@"SEL$1")
      OUTLINE(@"SEL$3")
      OUTLINE(@"SEL$2")
      MERGE(@"SEL$3" >"SEL$2")
      OUTLINE(@"SEL$335DD26A")
      OUTLINE(@"SEL$1")
      MERGE(@"SEL$335DD26A" >"SEL$1")
      OUTLINE(@"SEL$5C160134")
      OUTLINE(@"SEL$5")
      MERGE(@"SEL$5C160134" >"SEL$5")
      OUTLINE_LEAF(@"SEL$62A6D27E")
      OUTLINE_LEAF(@"SEL$4")
      ALL_ROWS
      DB_VERSION('18.1.0')
      OPTIMIZER_FEATURES_ENABLE('18.1.0')
      IGNORE_OPTIM_EMBEDDED_HINTS
      END_OUTLINE_DATA
  */

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter("X"."ADVISORYID"=100511)
   4 - filter("XID"="A"."VERSIONS_XID" AND ("CON_ID"=0 OR "CON_ID"=3))

And in addition a couple of my scripts for fixed tables:

https://github.com/xtender/xt_scripts/blob/master/fixed_tables.sql

https://github.com/xtender/xt_scripts/blob/master/fixed_indexes.sql

SQL: Split periods

Posted on by Posted in oracle, SQL 1,743 Page views Leave a comment

Some time ago there was a question on the sql.ru about splitting validity periods for gym cards to enabled and blocked periods:

-- we have a table with gym cards (card id, validity period: from/to)
CREATE TABLE cards(id_card NUMBER, num_card VARCHAR2(100), dt_from DATE, dt_to DATE);
-- periods when cards were blocked (record id, card id, dates from/to)
CREATE TABLE blocks(id_block NUMBER, id_card NUMBER, dt_from DATE, dt_to DATE);
-- cards data:
INSERT INTO cards (id_card, num_card, dt_from, dt_to) VALUES (1,'111111', TO_DATE('01.09.2016','DD.MM.YYYY'), TO_DATE('01.09.2019','DD.MM.YYYY'));
INSERT INTO cards (id_card, num_card, dt_from, dt_to) VALUES (2,'222222', TO_DATE('06.10.2017','DD.MM.YYYY'), TO_DATE('31.12.2050','DD.MM.YYYY'));
INSERT INTO cards (id_card, num_card, dt_from, dt_to) VALUES (3,'333333', TO_DATE('09.05.2019','DD.MM.YYYY'), TO_DATE('31.12.2050','DD.MM.YYYY'));
-- blocks data:
INSERT INTO blocks (id_block, id_card, dt_from, dt_to) VALUES (101, 1, TO_DATE('20.03.2017','DD.MM.YYYY'), TO_DATE('09.04.2017','DD.MM.YYYY'));
INSERT INTO blocks (id_block, id_card, dt_from, dt_to) VALUES (102, 1, TO_DATE('14.01.2018','DD.MM.YYYY'), TO_DATE('03.02.2018','DD.MM.YYYY'));
INSERT INTO blocks (id_block, id_card, dt_from, dt_to) VALUES (103, 1, TO_DATE('18.02.2019','DD.MM.YYYY'), TO_DATE('10.03.2019','DD.MM.YYYY'));
INSERT INTO blocks (id_block, id_card, dt_from, dt_to) VALUES (201, 2, TO_DATE('02.08.2018','DD.MM.YYYY'), TO_DATE('10.11.2018','DD.MM.YYYY'));
COMMIT;

Just for fun, I’ve solved it using match_recognise:

with 
 cards (id_card, num_card, dt_from, dt_to) as (
   select 1,'111111', TO_DATE('01.09.2016','DD.MM.YYYY'), TO_DATE('01.09.2019','DD.MM.YYYY') from dual union all
   select 2,'222222', TO_DATE('06.10.2017','DD.MM.YYYY'), TO_DATE('31.12.2050','DD.MM.YYYY') from dual union all
   select 3,'333333', TO_DATE('09.05.2019','DD.MM.YYYY'), TO_DATE('31.12.2050','DD.MM.YYYY') from dual 
),blocks (id_block, id_card, dt_from, dt_to) as (
   select 101, 1, TO_DATE('20.03.2017','DD.MM.YYYY'), TO_DATE('09.04.2017','DD.MM.YYYY') from dual union all
   select 102, 1, TO_DATE('14.01.2018','DD.MM.YYYY'), TO_DATE('03.02.2018','DD.MM.YYYY') from dual union all
   select 103, 1, TO_DATE('18.02.2019','DD.MM.YYYY'), TO_DATE('10.03.2019','DD.MM.YYYY') from dual union all
   select 201, 2, TO_DATE('02.08.2018','DD.MM.YYYY'), TO_DATE('10.11.2018','DD.MM.YYYY') from dual
)
,data as (
   select *
   from (
         select * 
         from (
            select id_card,dt_from,dt_to,'c' t from cards
            union all
            select id_card,dt_to,dt_from,'b' from blocks
         ) v
         --where v.id_card=1 -- here we can filter card ID if needed
        ) piv
   unpivot(dt for x in (dt_from as 'dt_from', dt_to as 'dt_to'))
)
select * 
from data
match_recognize (
   partition by id_card
   order by dt /* we can modify add "t" into "order by clause" to make it more deterministic */
   measures
      match_number() as N,
      first(dt) as dt_from,
      last(dt) as dt_to
   pattern (x_start+ x_end)
   define
      x_start as x = 'dt_from'
     ,x_end   as x = 'dt_to'
);

Workarounds for JPPD with view and table(kokbf$), xmltable or json_table functions

Posted on by Posted in CBO, oracle, query optimizing, SQL, troubleshooting 2,195 Page views Leave a comment

You may know that table() (kokbf$ collection functions), xmltable and json_table functions block Join-Predicate PushDown(JPPD).

Simple example:

DDL

[sourcecode language=”sql”]
create table xtest(a, b, c) as
select mod(level,1000),level,rpad(‘x’,100,’x’)
from dual
connect by level<=1e4
/
create index itest on xtest(a)
/
create or replace view vtest as
select a,count(b) cnt
from xtest
group by a
/
call dbms_stats.gather_table_stats(user,’xtest’);
/
[/sourcecode]

[collapse]

select distinct v.* 
from table(sys.odcinumberlist(1,2,3)) c, vtest v
where v.a = c.column_value;

Plan hash value: 699667151

-------------------------------------------------------------------------------------------------
| Id  | Operation                               | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                        |       |     1 |    19 |    80   (4)| 00:00:01 |
|   1 |  HASH UNIQUE                            |       |     1 |    19 |    80   (4)| 00:00:01 |
|*  2 |   HASH JOIN                             |       |     1 |    19 |    79   (3)| 00:00:01 |
|   3 |    COLLECTION ITERATOR CONSTRUCTOR FETCH|       |     1 |     2 |    29   (0)| 00:00:01 |
|   4 |    VIEW                                 | VTEST |  1000 | 17000 |    49   (3)| 00:00:01 |
|   5 |     HASH GROUP BY                       |       |  1000 |  8000 |    49   (3)| 00:00:01 |
|   6 |      TABLE ACCESS FULL                  | XTEST | 10000 | 80000 |    48   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("V"."A"=VALUE(KOKBF$))

same for json_table

[sourcecode language=”sql”]
select/*+ cardinality(c 1) use_nl(v) push_pred(v) */ *
from json_table(‘{"a":[1,2,3]}’, ‘$.a[*]’ COLUMNS (a int PATH ‘$’)) c
,vtest v
where c.a = v.a;

Plan hash value: 664523328

——————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
——————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 28 | 78 (2)| 00:00:01 |
| 1 | NESTED LOOPS | | 1 | 28 | 78 (2)| 00:00:01 |
| 2 | JSONTABLE EVALUATION | | | | | |
|* 3 | VIEW | VTEST | 1 | 26 | 49 (3)| 00:00:01 |
| 4 | SORT GROUP BY | | 1000 | 8000 | 49 (3)| 00:00:01 |
| 5 | TABLE ACCESS FULL | XTEST | 10000 | 80000 | 48 (0)| 00:00:01 |
——————————————————————————–

Predicate Information (identified by operation id):
—————————————————

3 – filter("V"."A"="P"."A")

Hint Report (identified by operation id / Query Block Name / Object Alias):
Total hints for statement: 1 (U – Unused (1))
—————————————————————————

1 – SEL$F534CA49 / V@SEL$1
U – push_pred(v)
[/sourcecode]

[collapse]
same for xmltable

[sourcecode language=”sql”]
select/*+ leading(c v) cardinality(c 1) use_nl(v) push_pred(v) */ v.*
from xmltable(‘(1,3)’ columns a int path ‘.’) c,vtest v
where c.a = v.a(+);

Plan hash value: 564839666

————————————————————————————————————
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
————————————————————————————————————
| 0 | SELECT STATEMENT | | 1 | 28 | 78 (2)| 00:00:01 |
| 1 | NESTED LOOPS OUTER | | 1 | 28 | 78 (2)| 00:00:01 |
| 2 | COLLECTION ITERATOR PICKLER FETCH| XQSEQUENCEFROMXMLTYPE | 1 | 2 | 29 (0)| 00:00:01 |
|* 3 | VIEW | VTEST | 1 | 26 | 49 (3)| 00:00:01 |
| 4 | SORT GROUP BY | | 1000 | 8000 | 49 (3)| 00:00:01 |
| 5 | TABLE ACCESS FULL | XTEST | 10000 | 80000 | 48 (0)| 00:00:01 |
————————————————————————————————————

Predicate Information (identified by operation id):
—————————————————

3 – filter("V"."A"(+)=CAST(TO_NUMBER(SYS_XQ_UPKXML2SQL(SYS_XQEXVAL(VALUE(KOKBF$),0,0,54525952,0),
50,1,2)) AS int ))

Hint Report (identified by operation id / Query Block Name / Object Alias):
Total hints for statement: 1 (U – Unused (1))
—————————————————————————

1 – SEL$6722A2F6 / V@SEL$1
U – push_pred(v)
[/sourcecode]

[collapse]

And compare with this:

create global temporary table temp_collection(a number);

insert into temp_collection select * from table(sys.odcinumberlist(1,2,3));

select/*+ cardinality(c 1) no_merge(v) */
   distinct v.* 
from temp_collection c, vtest v
where v.a = c.a;

Plan hash value: 3561835411

------------------------------------------------------------------------------------------------------------
| Id  | Operation                                | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                         |                 |     1 |    26 |    41   (3)| 00:00:01 |
|   1 |  HASH UNIQUE                             |                 |     1 |    26 |    41   (3)| 00:00:01 |
|   2 |   NESTED LOOPS                           |                 |     1 |    26 |    40   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL                     | TEMP_COLLECTION |     1 |    13 |    29   (0)| 00:00:01 |
|   4 |    VIEW PUSHED PREDICATE                 | VTEST           |     1 |    13 |    11   (0)| 00:00:01 |
|*  5 |     FILTER                               |                 |       |       |            |          |
|   6 |      SORT AGGREGATE                      |                 |     1 |     8 |            |          |
|   7 |       TABLE ACCESS BY INDEX ROWID BATCHED| XTEST           |    10 |    80 |    11   (0)| 00:00:01 |
|*  8 |        INDEX RANGE SCAN                  | ITEST           |    10 |       |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   5 - filter(COUNT(*)>0)
   8 - access("A"="C"."A")

You can see that JPPD works fine in case of global temporary tables and, obviously, the first workaround is to avoid such functions with complex views.
But in such simple queries you have 2 other simple options:
1. you can avoid JPPD and get optimal plans using CVM(complex view merge) by just simply rewriting the query using IN or EXISTS:

select * 
from vtest v
where v.a in (select/*+ cardinality(c 1) */ c.* from table(sys.odcinumberlist(1,2,3)) c);

Plan hash value: 1474391442

---------------------------------------------------------------------------------------------------
| Id  | Operation                                 | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                          |       |    10 |   100 |    42   (5)| 00:00:01 |
|   1 |  SORT GROUP BY NOSORT                     |       |    10 |   100 |    42   (5)| 00:00:01 |
|   2 |   NESTED LOOPS                            |       |    10 |   100 |    41   (3)| 00:00:01 |
|   3 |    NESTED LOOPS                           |       |    10 |   100 |    41   (3)| 00:00:01 |
|   4 |     SORT UNIQUE                           |       |     1 |     2 |    29   (0)| 00:00:01 |
|   5 |      COLLECTION ITERATOR CONSTRUCTOR FETCH|       |     1 |     2 |    29   (0)| 00:00:01 |
|*  6 |     INDEX RANGE SCAN                      | ITEST |    10 |       |     1   (0)| 00:00:01 |
|   7 |    TABLE ACCESS BY INDEX ROWID            | XTEST |    10 |    80 |    11   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   6 - access("A"=VALUE(KOKBF$))

the same with json_table and xmltable

[sourcecode language=”sql”]
select *
from vtest t
where t.a in (select/*+ cardinality(v 1) */ v.a from json_table(‘{"a":[1,2,3]}’, ‘$.a[*]’ COLUMNS (a int PATH ‘$’)) v);

Plan hash value: 2910004067

—————————————————————————————
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
—————————————————————————————
| 0 | SELECT STATEMENT | | 10 | 100 | 42 (5)| 00:00:01 |
| 1 | SORT GROUP BY NOSORT | | 10 | 100 | 42 (5)| 00:00:01 |
| 2 | NESTED LOOPS | | 10 | 100 | 41 (3)| 00:00:01 |
| 3 | NESTED LOOPS | | 10 | 100 | 41 (3)| 00:00:01 |
| 4 | SORT UNIQUE | | | | | |
| 5 | JSONTABLE EVALUATION | | | | | |
|* 6 | INDEX RANGE SCAN | ITEST | 10 | | 1 (0)| 00:00:01 |
| 7 | TABLE ACCESS BY INDEX ROWID| XTEST | 10 | 80 | 11 (0)| 00:00:01 |
—————————————————————————————

Predicate Information (identified by operation id):
—————————————————

6 – access("A"="P"."A")

select v.*
from vtest v
where exists(select/*+ cardinality(c 1) */ 1 from xmltable(‘(1,3)’ columns a int path ‘.’) c where c.a = v.a);

Plan hash value: 1646016183

—————————————————————————————————————
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
—————————————————————————————————————
| 0 | SELECT STATEMENT | | 10 | 100 | 42 (5)| 00:00:01 |
| 1 | SORT GROUP BY NOSORT | | 10 | 100 | 42 (5)| 00:00:01 |
| 2 | NESTED LOOPS | | 10 | 100 | 41 (3)| 00:00:01 |
| 3 | NESTED LOOPS | | 10 | 100 | 41 (3)| 00:00:01 |
| 4 | SORT UNIQUE | | 1 | 2 | 29 (0)| 00:00:01 |
| 5 | COLLECTION ITERATOR PICKLER FETCH| XQSEQUENCEFROMXMLTYPE | 1 | 2 | 29 (0)| 00:00:01 |
|* 6 | INDEX RANGE SCAN | ITEST | 10 | | 1 (0)| 00:00:01 |
| 7 | TABLE ACCESS BY INDEX ROWID | XTEST | 10 | 80 | 11 (0)| 00:00:01 |
—————————————————————————————————————

Predicate Information (identified by operation id):
—————————————————

6 – access("A"=CAST(TO_NUMBER(SYS_XQ_UPKXML2SQL(SYS_XQEXVAL(VALUE(KOKBF$),0,0,54525952,0),50,1,2)) AS int ))
[/sourcecode]

[collapse]

2. Avoid JPPD using lateral():

select/*+ cardinality(c 1) no_merge(lat) */
   distinct lat.* 
from table(sys.odcinumberlist(1,2,3)) c, 
     lateral(select * from vtest v where v.a = c.column_value) lat;

Plan hash value: 18036714

-----------------------------------------------------------------------------------------------------------
| Id  | Operation                               | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                        |                 |    10 |   190 |    41   (3)| 00:00:01 |
|   1 |  HASH UNIQUE                            |                 |    10 |   190 |    41   (3)| 00:00:01 |
|   2 |   NESTED LOOPS                          |                 |    10 |   190 |    40   (0)| 00:00:01 |
|   3 |    COLLECTION ITERATOR CONSTRUCTOR FETCH|                 |     1 |     2 |    29   (0)| 00:00:01 |
|   4 |    VIEW                                 | VW_LAT_4DB60E85 |    10 |   170 |    11   (0)| 00:00:01 |
|   5 |     SORT GROUP BY                       |                 |    10 |    80 |    11   (0)| 00:00:01 |
|   6 |      TABLE ACCESS BY INDEX ROWID BATCHED| XTEST           |    10 |    80 |    11   (0)| 00:00:01 |
|*  7 |       INDEX RANGE SCAN                  | ITEST           |    10 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   7 - access("A"=VALUE(KOKBF$))

Let’s see a bit more complex query:

Test tables 2

[sourcecode language=”sql”]
create table xtest1(id primary key, a) as
select level,level from dual connect by level<=1000;

create table xtest2(a, b, c) as
select mod(level,1000),level,rpad(‘x’,100,’x’)
from dual
connect by level<=1e4
/
create index itest2 on xtest2(a)
/
create or replace view vtest2 as
select a,count(b) cnt
from xtest2
group by a
/
[/sourcecode]

[collapse]

select v.* 
from xtest1 t1,
     vtest2 v
where t1.id in (select/*+ cardinality(c 1) */ * from table(sys.odcinumberlist(1,2,3)) c)
  and v.a = t1.a;

Plan hash value: 4293766070

-----------------------------------------------------------------------------------------------------------
| Id  | Operation                                  | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                           |              |     1 |    36 |    80   (3)| 00:00:01 |
|*  1 |  HASH JOIN                                 |              |     1 |    36 |    80   (3)| 00:00:01 |
|   2 |   JOIN FILTER CREATE                       | :BF0000      |     1 |    10 |    31   (4)| 00:00:01 |
|   3 |    NESTED LOOPS                            |              |     1 |    10 |    31   (4)| 00:00:01 |
|   4 |     NESTED LOOPS                           |              |     1 |    10 |    31   (4)| 00:00:01 |
|   5 |      SORT UNIQUE                           |              |     1 |     2 |    29   (0)| 00:00:01 |
|   6 |       COLLECTION ITERATOR CONSTRUCTOR FETCH|              |     1 |     2 |    29   (0)| 00:00:01 |
|*  7 |      INDEX UNIQUE SCAN                     | SYS_C0026365 |     1 |       |     0   (0)| 00:00:01 |
|   8 |     TABLE ACCESS BY INDEX ROWID            | XTEST1       |     1 |     8 |     1   (0)| 00:00:01 |
|   9 |   VIEW                                     | VTEST2       |  1000 | 26000 |    49   (3)| 00:00:01 |
|  10 |    HASH GROUP BY                           |              |  1000 |  8000 |    49   (3)| 00:00:01 |
|  11 |     JOIN FILTER USE                        | :BF0000      | 10000 | 80000 |    48   (0)| 00:00:01 |
|* 12 |      TABLE ACCESS FULL                     | XTEST2       | 10000 | 80000 |    48   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("V"."A"="T1"."A")
   7 - access("T1"."ID"=VALUE(KOKBF$))
  12 - filter(SYS_OP_BLOOM_FILTER(:BF0000,"A"))

As you see, CVM can’t help in this case, but we can use lateral():

select/*+ no_merge(lat) */ lat.* 
from xtest1 t1,
     lateral(select * from vtest2 v where v.a = t1.a) lat
where t1.id in (select/*+ cardinality(c 1) */ * from table(sys.odcinumberlist(1,2,3)) c);

Plan hash value: 1798023704

------------------------------------------------------------------------------------------------------------
| Id  | Operation                                | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                         |                 |    10 |   360 |    42   (3)| 00:00:01 |
|   1 |  NESTED LOOPS                            |                 |    10 |   360 |    42   (3)| 00:00:01 |
|   2 |   NESTED LOOPS                           |                 |     1 |    10 |    31   (4)| 00:00:01 |
|   3 |    SORT UNIQUE                           |                 |     1 |     2 |    29   (0)| 00:00:01 |
|   4 |     COLLECTION ITERATOR CONSTRUCTOR FETCH|                 |     1 |     2 |    29   (0)| 00:00:01 |
|   5 |    TABLE ACCESS BY INDEX ROWID           | XTEST1          |     1 |     8 |     1   (0)| 00:00:01 |
|*  6 |     INDEX UNIQUE SCAN                    | SYS_C0026365    |     1 |       |     0   (0)| 00:00:01 |
|   7 |   VIEW                                   | VW_LAT_A18161FF |    10 |   260 |    11   (0)| 00:00:01 |
|   8 |    SORT GROUP BY                         |                 |    10 |    80 |    11   (0)| 00:00:01 |
|   9 |     TABLE ACCESS BY INDEX ROWID BATCHED  | XTEST2          |    10 |    80 |    11   (0)| 00:00:01 |
|* 10 |      INDEX RANGE SCAN                    | ITEST2          |    10 |       |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   6 - access("T1"."ID"=VALUE(KOKBF$))
  10 - access("A"="T1"."A")

There is also another workaround with non-documented “precompute_subquery” hint:

select v.* 
from xtest1 t1,
     vtest2 v 
where t1.id in (select/*+ precompute_subquery */ * from table(sys.odcinumberlist(1,2,3)) c)
and v.a = t1.a;

Plan hash value: 1964829099

------------------------------------------------------------------------------------------------
| Id  | Operation                       | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                |              |    30 |   480 |    37   (3)| 00:00:01 |
|   1 |  HASH GROUP BY                  |              |    30 |   480 |    37   (3)| 00:00:01 |
|   2 |   NESTED LOOPS                  |              |    30 |   480 |    36   (0)| 00:00:01 |
|   3 |    NESTED LOOPS                 |              |    30 |   480 |    36   (0)| 00:00:01 |
|   4 |     INLIST ITERATOR             |              |       |       |            |          |
|   5 |      TABLE ACCESS BY INDEX ROWID| XTEST1       |     3 |    24 |     3   (0)| 00:00:01 |
|*  6 |       INDEX UNIQUE SCAN         | SYS_C0026365 |     3 |       |     2   (0)| 00:00:01 |
|*  7 |     INDEX RANGE SCAN            | ITEST2       |    10 |       |     1   (0)| 00:00:01 |
|   8 |    TABLE ACCESS BY INDEX ROWID  | XTEST2       |    10 |    80 |    11   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   6 - access("T1"."ID"=1 OR "T1"."ID"=2 OR "T1"."ID"=3)
   7 - access("A"="T1"."A")

It can help even in most difficult cases, for example if you can’t rewrite query (in this case you can create sql patch or sql profile with “precompute_subquery”), but I wouldn’t suggest it since “precompute_subquery” is non-documented, it can be used only with simple collections and has limitation in 1000 values.
I’d suggest to use the workaround with lateral, since it’s most reliable and very simple.

v$sql_hint.target_level

Posted on by Posted in CBO, oracle, SQL, undocumented 2,015 Page views Leave a comment

Today I wanted to give a link to the description of v$sql_hint.target_level to show that no_parallel can be specified for statement or object, and though it’s pretty obvious, but surprisingly I haven’t found any articles or posts about it, so this short post describes it.
v$sql_hint.target_level is a bitset, where
1st bit set to 1 means that the hint can be specified on statement level,
2nd – on query block level,
3rd – on object level,
4th – on join level(for multiple objects).
Short example:

   select name,sql_feature
          ,class,inverse
          ,version,version_outline
          ,target_level
         ,decode(bitand(target_level,1),0,'no','yes') Statement_level
         ,decode(bitand(target_level,2),0,'no','yes') Query_block_level
         ,decode(bitand(target_level,4),0,'no','yes') Object_level
         ,decode(bitand(target_level,8),0,'no','yes') Join_level
   from v$sql_hint h;

with hints as (
   select name,sql_feature
          ,class,inverse
          ,version,version_outline
          ,target_level
         ,decode(bitand(target_level,1),0,'no','yes') Statement_level
         ,decode(bitand(target_level,2),0,'no','yes') Query_block_level
         ,decode(bitand(target_level,4),0,'no','yes') Object_level
         ,decode(bitand(target_level,8),0,'no','yes') Join_level
   from v$sql_hint h
)
select *
from hints
where statement_level='yes'
  and to_number(regexp_substr(version,'^\d+')) >= 18
order by version;

Result:

NAME              SQL_FEATURE     CLASS                VERSION  TARGET_LEVEL STATEMENT_LEVEL QUERY_BLOCK_LEVEL OBJECT_LEVEL JOIN_LEVEL
----------------- --------------- -------------------- -------- ------------ --------------- ----------------- ------------ ----------
PDB_LOCAL_ONLY    QKSFM_DML       PDB_LOCAL_ONLY       18.1.0              1 yes             no                no           no
SUPPRESS_LOAD     QKSFM_DDL       SUPPRESS_LOAD        18.1.0              1 yes             no                no           no
SYSTEM_STATS      QKSFM_ALL       SYSTEM_STATS         18.1.0              1 yes             no                no           no
MEMOPTIMIZE_WRITE QKSFM_EXECUTION MEMOPTIMIZE_WRITE    18.1.0              1 yes             no                no           no
SKIP_PROXY        QKSFM_ALL       SKIP_PROXY           18.1.0              1 yes             no                no           no
CURRENT_INSTANCE  QKSFM_ALL       CURRENT_INSTANCE     18.1.0              1 yes             no                no           no
JSON_LENGTH       QKSFM_EXECUTION JSON_LENGTH          19.1.0              1 yes             no                no           no
QUARANTINE        QKSFM_EXECUTION QUARANTINE           19.1.0              1 yes             no                no           no

Top time-consuming predicates from ASH

Posted on by Posted in oracle, query optimizing, SQL, statistics, troubleshooting 1,883 Page views Leave a comment

Sometimes it might be useful to analyze top time-consuming filter and access predicates from ASH, especially in cases when db load is spread evenly enough by different queries and top segments do not show anything interesting, except usual things like “some tables are requested more often than others”.
Of course, we can start from analysis of SYS.COL_USAGE$: col_usage.sql

col_usage.sql

[sourcecode language=”sql”]
col owner format a30
col oname format a30 heading "Object name"
col cname format a30 heading "Column name"
accept owner_mask prompt "Enter owner mask: ";
accept tab_name prompt "Enter tab_name mask: ";
accept col_name prompt "Enter col_name mask: ";

SELECT a.username as owner
,o.name as oname
,c.name as cname
,u.equality_preds as equality_preds
,u.equijoin_preds as equijoin_preds
,u.nonequijoin_preds as nonequijoin_preds
,u.range_preds as range_preds
,u.like_preds as like_preds
,u.null_preds as null_preds
,to_char(u.timestamp, ‘yyyy-mm-dd hh24:mi:ss’) when
FROM
sys.col_usage$ u
, sys.obj$ o
, sys.col$ c
, all_users a
WHERE a.user_id = o.owner#
AND u.obj# = o.obj#
AND u.obj# = c.obj#
AND u.intcol# = c.col#
AND a.username like upper(‘&owner_mask’)
AND o.name like upper(‘&tab_name’)
AND c.name like upper(‘&col_name’)
ORDER BY a.username, o.name, c.name
;
col owner clear;
col oname clear;
col cname clear;
undef tab_name col_name owner_mask;
[/sourcecode]

[collapse]

But it’s not enough, for example it doesn’t show predicates combinations. In this case we can use v$active_session_history and v$sql_plan:

Top 50 predicates

[sourcecode language=”sql”]
with
ash as (
select
sql_id
,plan_hash_value
,table_name
,alias
,ACCESS_PREDICATES
,FILTER_PREDICATES
,count(*) cnt
from (
select
h.sql_id
,h.SQL_PLAN_HASH_VALUE plan_hash_value
,decode(p.OPERATION
,’TABLE ACCESS’,p.OBJECT_OWNER||’.’||p.OBJECT_NAME
,(select i.TABLE_OWNER||’.’||i.TABLE_NAME from dba_indexes i where i.OWNER=p.OBJECT_OWNER and i.index_name=p.OBJECT_NAME)
) table_name
,OBJECT_ALIAS ALIAS
,p.ACCESS_PREDICATES
,p.FILTER_PREDICATES
— поля, которые могут быть полезны для анализа в других разрезах:
— ,h.sql_plan_operation
— ,h.sql_plan_options
— ,decode(h.session_state,’ON CPU’,’ON CPU’,h.event) event
— ,h.current_obj#
from v$active_session_history h
,v$sql_plan p
where h.sql_opname=’SELECT’
and h.IN_SQL_EXECUTION=’Y’
and h.sql_plan_operation in (‘INDEX’,’TABLE ACCESS’)
and p.SQL_ID = h.sql_id
and p.CHILD_NUMBER = h.SQL_CHILD_NUMBER
and p.ID = h.SQL_PLAN_LINE_ID
— если захотим за последние 3 часа:
— and h.sample_time >= systimestamp – interval ‘3’ hour
)
— если захотим анализируем предикаты только одной таблицы:
— where table_name=’&OWNER.&TABNAME’
group by
sql_id
,plan_hash_value
,table_name
,alias
,ACCESS_PREDICATES
,FILTER_PREDICATES
)
,agg_by_alias as (
select
table_name
,regexp_substr(ALIAS,’^[^@]+’) ALIAS
,listagg(ACCESS_PREDICATES,’ ‘) within group(order by ACCESS_PREDICATES) ACCESS_PREDICATES
,listagg(FILTER_PREDICATES,’ ‘) within group(order by FILTER_PREDICATES) FILTER_PREDICATES
,sum(cnt) cnt
from ash
group by
sql_id
,plan_hash_value
,table_name
,alias
)
,agg as (
select
table_name
,’ALIAS’ alias
,replace(access_predicates,’"’||alias||’".’,’"ALIAS".’) access_predicates
,replace(filter_predicates,’"’||alias||’".’,’"ALIAS".’) filter_predicates
,sum(cnt) cnt
from agg_by_alias
group by
table_name
,replace(access_predicates,’"’||alias||’".’,’"ALIAS".’)
,replace(filter_predicates,’"’||alias||’".’,’"ALIAS".’)
)
,cols as (
select
table_name
,cols
,access_predicates
,filter_predicates
,sum(cnt)over(partition by table_name,cols) total_by_cols
,cnt
from agg
,xmltable(
‘string-join(for $c in /ROWSET/ROW/COL order by $c return $c,",")’
passing
xmltype(
cursor(
(select distinct
nvl(
regexp_substr(
access_predicates||’ ‘||filter_predicates
,'("’||alias||’"\.|[^.]|^)"([A-Z0-9#_$]+)"([^.]|$)’
,1
,level
,’i’,2
),’ ‘)
col
from dual
connect by
level<=regexp_count(
access_predicates||’ ‘||filter_predicates
,'("’||alias||’"\.|[^.]|^)"([A-Z0-9#_$]+)"([^.]|$)’
)
)
))
columns cols varchar2(400) path ‘.’
)(+)
order by total_by_cols desc, table_name, cnt desc
)
select
table_name
,cols
,sum(cnt)over(partition by table_name,cols) total_by_cols
,access_predicates
,filter_predicates
,cnt
from cols
where rownum<=50
order by total_by_cols desc, table_name, cnt desc;
[/sourcecode]

[collapse]

As you can see it shows top 50 predicates and their columns for last 3 hours. Despite the fact that ASH stores just sampled data, its results are representative enough for high-load databases.
Just few details:

  • Column “COLS” shows “search columns”, and total_by_cols – their number of occurrences
  • I think it’s obvious, that this info is not unambiguous marker of the problem, because for example few full table scans can misrepresent the statistics, so sometimes you will need to analyze such queries deeper (v$sqlstats,dba_hist_sqlstat)
  • We need to group data by OBJECT_ALIAS within SQL_ID and plan_hash_value, because in case of index access with lookup to table(“table access by rowid”) some predicates are in the row with index access and others are in the row with table access.

Depending on the needs, we can modify this query to analyze ASH data by different dimensions, for example with additional analysis of partitioning or wait events.

Correct syntax for the table_stats hint

Posted on by Posted in adaptive serial direct path reads, CBO, hints, oracle, SmartScan, trace, troubleshooting, undocumented 2,487 Page views 3 Comments

A friend contacted me to ask why they were having problems using the table_stats hint to influence optimizer decision making and also to influence the decision to use direct read or buffer cache scan so this is just a quick blog post to clarify the syntax as it is not well documented.

table_stats(<table_name> <method> {<keyword>=<value>} )

Method is one of: DEFAULT, SET, SCALE, SAMPLE

Keyword is one of: BLOCKS, ROWS, ROW_LENGTH
Continue reading→