An explain plan is a representation of the access path that is taken when a query is executed within Oracle.
Query processing can be divided into 7 phases:
| [1] Syntactic | Checks the syntax of the query |
| [2] Semantic | Checks that all objects exist and are accessible |
| [3] View Merging | Rewrites query as join on base tables as opposed to using views |
[4] Statement
Transformation | Rewrites
query transforming some complex constructs into simpler ones where
appropriate (e.g. subquery merging, in/or transformation) |
| [5] Optimization | Determines
the optimal access path for the query to take. With the Rule Based
Optimizer (RBO) it uses a set of heuristics to determine access path.
With the Cost Based Optimizer (CBO) we use statistics to analyze the
relative costs of accessing objects. |
| [6] QEP Generation | QEP = Query Evaluation Plan |
| [7] QEP Execution | QEP = Query Evaluation Plan |
Steps [1]-[6] are handled by the parser. Step [7] is the execution of the statement.
The
explain plan is produced by the parser. Once the access path has been
decided upon it is stored in the library cache together with the
statement itself. We store queries in the library cache based upon a
hashed representation of that query. When looking for a statement in
the library cache, we first apply a hashing algorithm to the statement
and then we look for this hash value in the library cache. This access
path will be used until the query is reparsed.
Terminology
| Row Source | A set of rows used in a query may be a select from a base object or the result set returned by joining 2 earlier row sources |
| Predicate | where clause of a query |
| Tuples | rows |
| Driving Table | This
is the row source that we use to seed the query. If this returns a lot
of rows then this can have a negative affect on all subsequent
operations |
| Probed Table | This is the object we lookup data in after we have retrieved relevant key data from the driving table.
|
How does Oracle access data?
At
the physical level Oracle reads blocks of data. The smallest amount of
data read is a single Oracle block, the largest is constrained by
operating system limits (and multiblock i/o). Logically Oracle finds the
data to read by using the following methods:
Explain plan Hierarchy
Query Plan
-----------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1234
TABLE ACCESS FULL LARGE [:Q65001] [ANALYZED]
The
rightmost uppermost operation of an explain plan is the first thing
that the explain plan will execute. In this case TABLE ACCESS FULL LARGE
is the first operation. This statement means we are doing a full table
scan of table LARGE. When this operation completes then the resultant
row source is passed up to the
next level of the query for processing. In this case it is the SELECT STATEMENT which is the top of the query.
[CHOOSE]
is an indication of the optimizer_goal for the query. This DOES NOT
necessarily indicate that plan has actually used this goal. The only way
to confirm this is to check the
cost=
part of the explain plan as well. For example the following query
indicates that the CBO has been used because there is a cost in the cost
field:
SELECT STATEMENT [CHOOSE] Cost=1234
However the explain plan below indicates the use of the RBO because the cost field is blank:
SELECT STATEMENT [CHOOSE] Cost=
The
cost field is a comparative cost that is used internally to determine
the best cost for particular plans. The costs of different statements
are not really directly comparable.
[:Q65001]
indicates that this particular part of the query is being executed in
parallel. This number indicates that the operation will be processed by a
parallel query slave as opposed to being executed serially.
[ANALYZED]
indicates that the object in question has been analyzed and there are
currently statistics available for the CBO to use. There is no
indication of the 'level' of analysis done.
Access Methods in detail
This defaults to:
db_block_buffers / ( (PROCESSES+3) / 4 )
Maximum values are OS dependant
Buffers
from FTS operations are placed on the Least Recently Used (LRU) end of
the buffer cache so will be quickly aged out. FTS is not recommended for
large tables unless you are reading >5-10% of it (or so) or you
intend to run in parallel.
Example FTS explain plan:
SQL> explain plan for select * from dual;
Query Plan
-----------------------------------------
SELECT STATEMENT [CHOOSE] Cost=
TABLE ACCESS FULL DUAL
Data
is accessed by looking up key values in an index and returning rowids. A
rowid uniquely identifies an individual row in a particular data block.
This block is read via single block i/o.
In
this example an index is used to find the relevant row(s) and then the
table is accessed to lookup the ename column (which is not included in
the index):
SQL> explain plan for
select empno,ename from emp where empno=10;
Query Plan
------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
TABLE ACCESS BY ROWID EMP [ANALYZED]
INDEX UNIQUE SCAN EMP_I1
Notice
the 'TABLE ACCESS BY ROWID' section. This indicates that the table data
is not being accessed via a FTS operation but rather by a rowid lookup.
In this case the rowid has been produced by looking up values in the
index first. The index is being accessed by an 'INDEX UNIQUE SCAN'
operation. This is explained below. The index name in this case is
EMP_I1. If all the required data resides in the index then a table
lookup may be unnecessary and all you will see is an index access with
no table access.
In the following example all the columns (empno) are in the index. Notice that no table access takes place:
SQL> explain plan for
select empno from emp where empno=10;
Query Plan
------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
INDEX UNIQUE SCAN EMP_I1
Indexes are presorted so sorting may be unecessary if the sort order required is the same as the index.
SQL> explain plan for select empno,ename from emp
where empno > 7876 order by empno;
Query Plan
-------------------------------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
TABLE ACCESS BY ROWID EMP [ANALYZED]
INDEX RANGE SCAN EMP_I1 [ANALYZED]
In this case the index is sorted so ther rows will be returned in the order of the index hence a sort is unecessary.
SQL> explain plan for
select /*+ Full(emp) */ empno,ename from emp
where empno> 7876 order by empno;
Query Plan
-------------------------------------------------------------
SELECT STATEMENT [CHOOSE] Cost=9
SORT ORDER BY
TABLE ACCESS FULL EMP [ANALYZED] Cost=1 Card=2 Bytes=66
Because we have forced a FTS the data is unsorted and so we must sort the data
after it has been retrieved.
There are 4 methods of index lookup:
index unique scan
index range scan
index full scan
index fast full scan
Method
for looking up a single key value via a unique index. Always returns a
single value You must supply AT LEAST the leading column of the index to
access data via the index, However this may return > 1 row as the
uniqueness will not be guaranteed.
SQL> explain plan for
select empno,ename from emp where empno=10;
Query Plan
------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
TABLE ACCESS BY ROWID EMP [ANALYZED]
INDEX UNIQUE SCAN EMP_I1
Method
for accessing multiple column values You must supply AT LEAST the
leading column of the index to access data via the index Can be used for
range operations (e.g. > < <> >= <= between)
SQL> explain plan for select empno,ename from emp
where empno > 7876 order by empno;
Query Plan
-------------------------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
TABLE ACCESS BY ROWID EMP [ANALYZED]
INDEX RANGE SCAN EMP_I1 [ANALYZED]
A non-unique index may return multiple values for the predicate col1 = 5 and will use an index range scan
SQL> explain plan for select mgr from emp where mgr = 5
Query plan
--------------------
SELECT STATEMENT [CHOOSE] Cost=1
INDEX RANGE SCAN EMP_I2 [ANALYZED]
In
certain circumstances it is possible for the whole index to be scanned
as opposed to a range scan (i.e. where no constraining predicates are
provided for a table). Full index scans are only available in the CBO
as otherwise we are unable to determine whether a full scan would be a
good idea or not. We choose an index Full Scan when we have statistics
that indicate that it is going to be more efficient than a Full table
scan and a sort.
For
example we may do a Full index scan when we do an unbounded scan of an
index and want the data to be ordered in the index order. The optimizer
may decide that selecting all the information from the index and not
sorting is more efficient than doing a FTS or a Fast Full Index Scan and
then sorting.
An
Index full scan will perform single block i/o's and so it may prove to
be inefficient. Index BE_IX is a concatenated index on big_emp
(empno,ename)
SQL> explain plan for select empno,ename
from big_emp order by empno,ename;
Query Plan
------------------------------------------------------------
SELECT STATEMENT [CHOOSE] Cost=26
INDEX FULL SCAN BE_IX [ANALYZED]
Scans
all the block in the index Rows are not returned in sorted order
Introduced in 7.3 and requires V733_PLANS_ENABLED=TRUE and CBO may be
hinted using INDEX_FFS hint uses multiblock i/o can be executed in
parallel can be used to access second column of concatenated indexes.
This is because we are selecting all of the index.
Note
that INDEX FAST FULL SCAN is the mechinism behind fast index create and
recreate. Index BE_IX is a concatenated index on big_emp (empno,ename)
SQL> explain plan for select empno,ename from big_emp;
Query Plan
------------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
INDEX FAST FULL SCAN BE_IX [ANALYZED]
Selecting the 2nd column of concatenated index:
SQL> explain plan for select ename from big_emp;
Query Plan
------------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
INDEX FAST FULL SCAN BE_IX [ANALYZED]
This
is the quickest access method available Oracle simply retrieves the
block specified and extracts the rows it is interested in. Most
frequently seen in explain plans as Table access by Rowid
SQL> explain plan for select * from dept where rowid = ':x';
Query Plan
------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
TABLE ACCESS BY ROWID DEPT [ANALYZED]
Table is accessed by rowid following index lookup:
SQL> explain plan for
select empno,ename from emp where empno=10;
Query Plan
------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
TABLE ACCESS BY ROWID EMP [ANALYZED]
INDEX UNIQUE SCAN EMP_I1
A
Join is a predicate that attempts to combine 2 row sources We only ever
join 2 row sources together Join steps are always performed serially
even though underlying row sources may have been accessed in parallel.
Join order - order in which joins are performed
The
join order makes a significant difference to the way in which the query
is executed. By accessing particular row sources first, certain
predicates may be satisfied that are not satisfied by with other join
orders. This may prevent certain access paths from being taken.
Suppose there is a concatenated index on A(a.col1,a.col2). Note that a.col1 is the leading column. Consider the following query:
select A.col4
from A,B,C
where B.col3 = 10
and A.col1 = B.col1
and A.col2 = C.col2
and C.col3 = 5
We could represent the joins present in the query using the following schematic:
B <---> A <---> C
col3=10 col3=5
There
are really only 2 ways we can drive the query: via B.col3 or C.col3. We
would have to do a Full scan of A to be able to drive off it. This is
unlikely to be efficient with large tables;
If
we drive off table B, using predicate B.col3=10 (as a filter or lookup
key) then we will retrieve the value for B.col1 and join to A.col1.
Because we have now filled the leading column of the concatenated index
on table A we can use this index to give us values for A.col2 and join
to A.
However
if we drive of table c, then we only get a value for a.col2 and since
this is a trailing column of a concatenated index and the leading column
has not been supplied at this point, we cannot use the index on a to
lookup the data.
So
it is likely that the best join order will be B A C. The CBO will
obviously use costs to establish whether the individual access paths are
a good idea or not.
If the CBO does not choose this join order then we can hint it by changing the from
clause to read:
from B,A,C
and using the /*+ ordered */ hint. The resultant query would be:
select /*+ ordered */ A.col4
from B,A,C
where B.col3 = 10
and A.col1 = B.col1
and A.col2 = C.col2
and C.col3 = 5
Join Types
Sort Merge Join (SMJ)
Nested Loops (NL)
Hash Join
Rows
are produced by Row Source 1 and are then sorted Rows from Row Source 2
are then produced and sorted by the same sort key as Row Source 1. Row
Source 1 and 2 are NOT accessed concurrently Sorted rows from both sides
are then merged together (joined)
MERGE
/ \
SORT SORT
| |
Row Source 1 Row Source 2
If
the row sources are already (known to be) sorted then the sort
operation is unecessary as long as both 'sides' are sorted using the
same key. Presorted row sources include indexed columns and row sources
that have already been sorted in earlier steps. Although the merge of
the 2 row sources is handled serially, the row sources could be accessed
in parallel.
SQL> explain plan for
select /*+ ordered */ e.deptno,d.deptno
from emp e,dept d
where e.deptno = d.deptno
order by e.deptno,d.deptno;
Query Plan
-------------------------------------
SELECT STATEMENT [CHOOSE] Cost=17
MERGE JOIN
SORT JOIN
TABLE ACCESS FULL EMP [ANALYZED]
SORT JOIN
TABLE ACCESS FULL DEPT [ANALYZED]
Sorting
is an expensive operation, especially with large tables. Because of
this, SMJ is often not a particularly efficient join method.
First we return all the rows from row source 1 Then we probe row source 2 once for each row returned from row source 1
Row source 1
~~~~~~~~~~~~
Row 1 -------------- -- Probe -> Row source 2
Row 2 -------------- -- Probe -> Row source 2
Row 3 -------------- -- Probe -> Row source 2
Row source 1 is known as the outer table
Row source 2 is known as the inner table
Accessing
row source 2 is known a probing the inner table For nested loops to be
efficient it is important that the first row source returns as few rows
as possible as this directly controls the number of probes of the second
row source. Also it helps if the access method for row source 2 is
efficient as this operation is being repeated once for every row
returned by row source 1.
SQL> explain plan for
select a.dname,b.sql
from dept a,emp b
where a.deptno = b.deptno;
Query Plan
-------------------------
SELECT STATEMENT [CHOOSE] Cost=5
NESTED LOOPS
TABLE ACCESS FULL DEPT [ANALYZED]
TABLE ACCESS FULL EMP [ANALYZED]
New
join type introduced in 7.3 More efficient in theory than NL & SMJ
Only accessible via the CBO Smallest row source is chosen and used to
build a hash table and a bitmap The second row source is hashed and
checked against the hash table looking for joins. The bitmap is used as a
quick lookup to check if rows are in the hash table and are especially
useful when the hash table is too large to fit in memory.
SQL> explain plan for select /*+ use_hash(emp) */ empno
from emp,dept where emp.deptno = dept.deptno;
Query Plan
---------------------------------
SELECT STATEMENT [CHOOSE] Cost=3
HASH JOIN
TABLE ACCESS FULL DEPT
TABLE ACCESS FULL EMP
Hash joins are enabled by the parameter HASH_JOIN_ENABLED=TRUE in the init.ora or session. TRUE is the default in 7.3
A
Cartesian Product is done where they are no join conditions between 2
row sources and there is no alternative method of accessing the data Not
really a join as such as there is no join! Typically this is caused by a
coding mistake where a join has been left out. It can be useful in some
circumstances - Star joins uses cartesian products.
Notice that there is no join between the 2 tables:
SQL> explain plan for
select emp.deptno,dept,deptno
from emp,dept
Query Plan
------------------------------
SLECT STATEMENT [CHOOSE] Cost=5
MERGE JOIN CARTESIAN
TABLE ACCESS FULL DEPT
SORT JOIN
TABLE ACCESS FULL EMP
The CARTESIAN keyword indicate that we are doing a cartesian product.
Operations
Operations that show up in explain plans
There are a number of different operations that promote sorts
order by clauses
group by
sort merge join
Note that if the row source is already appropriately sorted then no sorting is required. This is now indicated in 7.3:
SORT GROUP BY NOSORT
INDEX FULL SCAN .....
In this case the group by operation simply groups the rows it does not do the sort operation as this has already been completed.
Sorts
are expensive operations especially on large tables where the rows do
not fit in memory and spill to disk. By default sort blocks are placed
into the buffer cache. This may result in aging out of other blocks that
may be reread by other processes. To avoid this you can use the
parameter <Parameter:SORT_DIRECT_WRITES> which does not place sort blocks into the buffer cache.
Has
a number of different meanings used to indicate partition elimination
may also indicate an actual filter step where one row source is
filtering another functions such as min may introduce filter steps into
query plans
In
this example there are 2 filter steps. The first is effectively like a
NL except that it stops when it gets something that it doesn't like
(i.e. a bounded NL). This is there because of the not in. The second is
filtering out the min value:
SQL> explain plan for select * from emp
where empno not in (select min(empno)
from big_emp group by empno);
Query Plan
------------------
SELECT STATEMENT [CHOOSE] Cost=1
FILTER **** This is like a bounded nested loops
TABLE ACCESS FULL EMP [ANALYZED]
FILTER **** This filter is introduced by the min
SORT GROUP BY NOSORT
INDEX FULL SCAN BE_IX
This
example is also interesting in that it has a NOSORT function. The group
by does not need to sort because the index row source is already pre
sorted.
When
a view cannot be merged into the main query you will often see a
projection view operation. This indicates that the 'view' will be
selected from directly as opposed to being broken down into joins on the
base tables. A number of constructs make a view non mergeable. Inline
views are also non mergeable.
In the following example the select contains an inline view which cannot be merged:
SQL> explain plan for
select ename,tot
from emp,
(select empno,sum(empno) tot from big_emp group by empno) tmp
where emp.empno = tmp.empno;
Query Plan
------------------------
SELECT STATEMENT [CHOOSE]
HASH JOIN
TABLE ACCESS FULL EMP [ANALYZED]
VIEW
SORT GROUP BY
INDEX FULL SCAN BE_IX
In
this case the inline view tmp which contains an aggregate function
cannot be merged into the main query. The explain plan shows this as a
view step.
Allows
a large table to be broken up into a number of smaller partitions which
can be queried much more quickly than the table as a whole a union all
view is built over the top to provide the original functionality Check
constraints or where clauses provide partition elimination capabilities
SQL> explain plan for
select /*+ use_nl(p1,kbwyv1) ordered */ sum(prc_pd)
from parent1 p1, kbwyv1
where p1.class = 22
and kbwyv1.bitm_numb = p1.bitm_numb
and kbwyv1.year = 1997
and kbwyv1.week between 32 and 33 ;
Query Plan
-----------------------------------------
SELECT STATEMENT [FIRST_ROWS] Cost=1780
SORT AGGREGATE
NESTED LOOPS [:Q65001] Ct=1780 Cd=40 Bt=3120
TABLE ACCESS FULL PARENT1 [:Q65000] [AN] Ct=20 Cd=40 Bt=1040
VIEW KBWYV1 [:Q65001]
UNION-ALL PARTITION [:Q65001]
FILTER [:Q64000]
TABLE ACCESS FULL KBWYT1 [AN] Ct=11 Cd=2000 Bt=104000
TABLE ACCESS FULL KBWYT2 [AN] Ct=11 Cd=2000 Bt=104000
TABLE ACCESS FULL KBWYT3 [AN] Ct=11 Cd=2000 Bt=104000
FILTER [:Q61000]
TABLE ACCESS FULL KBWYT4 [AN] Ct=11 Cd=2000 Bt=104000
KBWYV1
is a view on 4 tables KBWYT1-4. KBWYT1-4 contain rows for week 31-34
respectively and are maintained by check constraints. This query should
only return rows from partions 2 & 3. The filter operation indicates
this. Partitions 1 & 4 are eliminated at execution time. The view
line indicates that the view is not merged. The union-all partion
information indicates that we have recognised this as a partition view.
Note that the tables can be accessed in parallel.
Remote Queries
Only
shows remote in the OPERATION column OTHER column shows query executed
on remote node OTHER_NODE shows where it is executed Different
operational characteristics for RBO & CBO
RBO - Drags everything across the link and joins locally
CBO - Uses cost estimates to determine whether to execute remotely or locally
SQL> explain plan for
select *
from dept@loop_link;
Query Plan
-------------------------------------------------------
SELECT STATEMENT REMOTE [CHOOSE] Cost=1
TABLE ACCESS FULL DEPT [SJD.WORLD] [ANALYZED]
In this case the whole query has been sent to the remote site. The other column shows nothing.
SQL> explain plan for
select a.dname,avg(b.sal),max(b.sal)
from dept@loop_link a, emp b
where a.deptno=b.deptno
group by a.dname
order by max(b.sal),avg(b.sal) desc;
Query Plan
-----------------------------------------------------
SELECT STATEMENT [CHOOSE] Cost=20
SORT ORDER BY [:Q137003] [PARALLEL_TO_SERIAL]
SORT GROUP BY [:Q137002] [PARALLEL_TO_PARALLEL]
NESTED LOOPS [:Q137001] [PARALLEL_TO_PARALLEL]
REMOTE [:Q137000] [PARALLEL_FROM_SERIAL]
TABLE ACCESS FULL EMP [:Q137001] [ANALYZED]
[PARALLEL_COMBINED_WITH_PARENT]
Bind Variables
Bind variables are recommended in most cases because they promote sharing of sql code
At
parse time the parser has NO IDEA what the bind variable contains. With
RBO this makes no difference but with CBO, which relies on accurate
statistics to produce plans, this can be a problem.
Defining bind variables in sqlplus:
variable x varchar2(18);
assigning values:
begin
:x := 'hello';
end;
/
SQL> explain plan for
select *
from dept
where rowid = ':x';
Query Plan
------------------------------------
SELECT STATEMENT [CHOOSE] Cost=1
TABLE ACCESS BY ROWID DEPT [ANALYZED]
Parallel Query
Main indicators that a query is using PQO:
[:Q1000004] entries in the explain plan
Checkout the other column for details of what the slaves are executing
v$pq_slave will show any parallel activity
Columns to look in for information
other - contains the query passed to the slaves
other_tag - describes the contents of other
object_node - indicates order of pqo slaves
Parallel
Query operates on a producer/consumer basis. When you specify parallel
degree 4 oracle tries to allocate 4 producer slaves and 4 consumer
slaves. The producers can feed any of the consumers. If there are only 2
slaves available then we use these. If there is only 1 slave available
then we go serial If there are none available then we use serial. If
parallel_min_percent is set then we error ora 12827 instead of using a
lower number of slaves or going serial.
Consumer
processes typically perform a sorting function. If there is no
requirement for the data to be sorted then the consumer slaves are not
produced and we end up with the number of slaves used matching the
degree of parallelism as opposed to being 2x the degree.
Parallel Terms
| PARALLEL_FROM_SERIAL | This means that source of the data is serial but it is passed to a parallel consumer |
| PARALLEL_TO_PARALLEL | Both the consumer and the producer are parallel |
| PARALLEL_COMBINED_WITH_PARENT | This
operation has been combined with the parent operator. For example in a
sort merge join the sort operations would be shown as
PARALLEL_COMBINED_WITH_PARENT because the sort and the merge are handled
as 1 operation. |
| PARALELL_TO_SERIAL | The
source of the data is parallel but it is passed to a serial consumer.
This typically will happen at the top of the explain plan but could
occur anywhere
|
Assumptions
OPTIMIZER_MODE = CHOOSE
DEPT is small compared to EMP
DEPT has an index (DEPT_INDX) on deptno column
Three examples are presented
Query #1: Serial
Query #2: Parallel
Query #3: Parallel, with forced optimization to RULE and forced usage of DEPT_INDX
Sample Query #1 (Serial)
select A.dname, avg(B.sal), max(B.sal)
from dept A, emp B
where A.deptno = B.deptno
group by A.dname
order by max(B.sal), avg(B.sal) desc;
Execution Plan #1 (Serial)
OBJECT_NAME OBJECT_NODE OTHER
------------------------------- ----------- -------
SELECT STATEMENT
SORT ORDER BY
SORT GROUP BY
MERGE JOIN
SORT JOIN
TABLE ACCESS FULL emp
SORT JOIN
TABLE ACCESS FULL dept
Notice that the object_node and other columns are empty
Sample Query #2 (Query #1 with parallel hints)
select /*+ parallel(B,4) parallel(A,4) */
A.dname, avg(B.sal), max(B.sal)
from dept A, emp B
where A.deptno = B.deptno
group by A.dname
order by max(B.sal), avg(B.sal) desc;
Execution Plan #2 (Parallel)
OBJECT_NAME OBJECT_NODE OTHER
------------------------------- ----------- -------
SELECT STATEMENT Cost = ??
SORT ORDER BY :Q55004 **[7]**
SORT GROUP BY :Q55003 **[6]**
MERGE JOIN :Q55002 **[5]**
SORT JOIN :Q55002 **[4]**
TABLE ACCESS FULL emp :Q55001 **[2]**
SORT JOIN :Q55002 **[3]**
TABLE ACCESS FULL dept :Q55000 **[1]**
Execution Plan #2 -- OTHER column
**[1]** (:Q55000) "PARALLEL_FROM_SERIAL"
Serial execution of SELECT DEPTNO, DNAME FROM DEPT
**[2]** (:Q55001) "PARALLEL_TO_PARALLEL"
SELECT /*+ ROWID(A1)*/
A1."DEPTNO" C0, A1."SAL" C1
FROM "EMP" A1
WHERE ROWID BETWEEN :1 AND :2
**[3]** (:Q55002) "PARALLEL_COMBINED_WITH_PARENT"
**[4]** (:Q55002) "PARALLEL_COMBINED_WITH_PARENT"
**[5]** (:Q55002) "PARALLEL_TO_PARALLEL"
SELECT /*+ ORDERED USE_MERGE(A2)*/
A2.C1 C0, A1.C1 C1
FROM :Q55001 A1,:Q55000 A2
WHERE A1.C0=A2.C0
**[6]** (:Q55003) "PARALLEL_TO_PARALLEL"
SELECT MAX(A1.C1) C0, AVG(A1.C1) C1, A1.C0 C2
FROM :Q55002 A1
GROUP BY A1.C0
**[7]** (:Q55004) "PARALLEL_FROM_SERIAL"
SELECT A1.C0 C0, A1.C1 C1, A1.C2 C2
FROM :Q55003 A1
ORDER BY A1.CO, A1.C1 DESC
Sample Query #3 (Query #2 with fudged hints)
select /*+ index(A dept_indx) parallel(B,4) parallel(A,4) */
A.dname, avg(B.sal), max(B.sal)
from dept A, emp B
where A.deptno = B.deptno
group by A.dname
order by max(B.sal), avg(B.sal) desc;
Execution Plan #3 (Parallel)
OBJECT_NAME OBJECT_NODE OTHER
----------------------------------- ----------- -------
SELECT STATEMENT Cost = ??
SORT ORDER BY :Q58002 **[6]**
SORT GROUP BY :Q58001 **[5]**
NESTED LOOPS JOIN :Q58000 **[4]**
TABLE ACCESS FULL emp :Q58000 **[3]**
TABLE ACCESS BY ROWID dept :Q58000 **[2]**
INDEX RANGE SCAN dept_indx :Q58000 **[1]**
Execution Plan #3 -- OTHER column
**[1]** (:Q58000) "PARALLEL_COMBINED_WITH_PARENT"
**[2]** (:Q58000) "PARALLEL_COMBINED_WITH_PARENT"
**[3]** (:Q58000) "PARALLEL_COMBINED_WITH_PARENT"
**[4]** (:Q58000) "PARALLEL_TO_PARALLEL"
SELECT /*+ ORDERED USE_NL(A2) INDEX(A2) */
A2."DNAME" C0, A1.C0 C1
FROM
(SELECT /*+ ROWID(A3) */
A3."SAL" CO, A3."DEPTNO" C1
FROM "EMP" A3
WHERE ROWID BETWEEN :1 AND :2) A1,
"DEPT" A2
WHERE A2."DEPTNO" = A1.C1
**[5]** (:Q58001) "PARALLEL_TO_PARALLEL"
SELECT MAX(A1.C1) C0, AVG(A1.C1) C1, A1.C0 C2
FROM :Q58000 A1
GROUP BY A1.C0
**[6]** (:Q58002) "PARALLEL_TO_SERIAL"
SELECT A1.C0 C0, A1.C1 C1, A1.C2 C2
FROM :Q58001 A1
ORDER BY A1.C0, A1.C1 DESC
The following is a sample execution plan.
SQL> explain plan for
2 select e.empno, e.ename, d.dname
3 from emp e, dept d
4 where e.deptno = d.deptno
5 and e.deptno = 10;
Explained.
SQL> SELECT * FROM table(dbms_xplan.display(null,null,'basic'));
PLAN_TABLE_OUTPUT
------------------------------------------------------------
Plan hash value: 568005898
------------------------------------------------------------
| Id | Operation | Name |
------------------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | NESTED LOOPS | |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT |
| 3 | INDEX UNIQUE SCAN | PK_DEPT |
| 4 | TABLE ACCESS FULL | EMP |
------------------------------------------------
Using the rules above, you could say;
Operation 0 is the root of the tree; it has one child, Operation 1
Operation 1 has two children, which is Operation 2 and 4
Operation 2 has one child, which is Operation 3
Below
is the graphical representation of the execution plan. If you read the
tree; In order to perform Operation 1, you need to perform Operation 2
and 4. Operation 2 comes first; In order to perform 2, you need to
perform its Child Operation 3. In order to perform Operation 4, you need
to perform Operation 2
Operation 0
(SELECT STATEMENT)
|
| |
Operation 1
(NESTED LOOPS)
/ \
/ \
/ \
/ \
/ \
/ \
Operation 2 Operation 4
(TABLE ACCESS (TABLE ACCESS FULL)
BY INDEX ROWID)
|
|
|
Operation 3
(INDEX UNIQUE SCAN)
Operation 3 accesses DEPT table using INDEX UNIQUE SCAN and passes the ROWID to Operation 2
Operation 2 returns all the rows from DEPT table to Operation 1
Operation 1 performs Operation 4 for each row returned by Operation 2
Operation
4 performs a full table scan (TABLE ACCESS FULL) scan and applies the
filter E.DEPTNO=10 and returns the rows to Operation 1
Operation 1 returns the final results to Operation 0
