I've been playing with Oracle Spatial quite a bit lately and ran into some interesting things that I plan to write about in the next couple of posts.
This post covers an important consideration, I think, one should make when using SDO_NN spatial operator with sdo_num_res parameter.
This post covers an important consideration, I think, one should make when using SDO_NN spatial operator with sdo_num_res parameter.
But first, just briefly on the operator itself. SDO_NN operator is used to identify the
nearest neighbors for a particular geometry. The full
operator synopsis can be found in the official documentation
Please note, the database I use in the examples below is Oracle 11.2.0.3.0 Enterprise Edition with no additional patches applied and default Oracle Spatial settings.
Let's consider the following example,
there is a table RESTAURANTS having only an id and a geometry column
(point).
CREATE TABLE restaurants
(
id NUMBER NOT NULL,
point MDSYS.SDO_GEOMETRY
);
ALTER TABLE restaurants ADD CONSTRAINT restaurants_pk PRIMARY KEY (id);
Let's create a spatial index on
restaruants.point column:
INSERT INTO user_sdo_geom_metadata
( TABLE_NAME
, COLUMN_NAME
, DIMINFO
, SRID)
VALUES
( 'restaurants'
, 'POINT'
, SDO_DIM_ARRAY(
SDO_DIM_ELEMENT('X', 0, 10000000, 0.005),
SDO_DIM_ELEMENT('Y', 0, 10, 0.005)
)
, null -- SRID
);
CREATE INDEX restaurants_sidx ON restaurants(point) INDEXTYPE IS MDSYS.SPATIAL_INDEX;
For simplicity let's say there are
500,000 restaurants distributed on a single line (x-axis of two
dimensional coordinate system, y=0, x=1,2,3,...500,000).
INSERT INTO restaurants
(SELECT rownum --id
, SDO_GEOMETRY(
2001 -- 2 dimensional point
, null -- SDO SRID
, SDO_POINT_TYPE(rownum,0,null) –-x=rownum, y=0
, null
, null
)
FROM dual CONNECT BY rownum<=500000
);
In order to find the first 5 nearest
neighbors to point (10,0) one could use the following spatial
query:
SQL> SELECT p.id, sdo_nn_distance(1) distance
2 FROM restaurants p
3 WHERE SDO_NN(p.point,
4 SDO_GEOMETRY(
5 2001 -- 2 dimensional point
6 , null
7 , SDO_POINT_TYPE(10,0,null)
8 , null
9 , null
10 ),
11 'sdo_num_res=5',
12 1
13 )='TRUE'
14* ORDER BY 2;
ID DISTANCE
---------- ----------
10 0
9 1
11 1
12 2
8 2
SQL>
Now suppose each restaurant has 5 menu
items that are stored in MENU_ITEMS table. The table has 4 columns, a
unique identifier ID; a reference to RESTAURANTS table,
rest_id; an identifier that identifies an item within the
restaurant menu, item_number; and lastly a description, 100
characters long column that I use in this example to make this table
a bit bigger.
CREATE TABLE menu_items
(id NUMBER NOT NULL,
rest_id NUMBER NOT NULL,
item_number NUMBER NOT NULL,
description varchar2(100)
);
ALTER TABLE menu_items ADD CONSTRAINT menu_items_pk PRIMARY KEY (id);
ALTER TABLE menu_items ADD CONSTRAINT menu_items_rest_FK FOREIGN KEY (rest_id) REFERENCES restaurants(id);
CREATE INDEX menu_items_rest_idx ON menu_items(rest_id);
INSERT INTO menu_items
(SELECT rownum
, b.rn
, a.rn
, rpad(rownum,100,'A')
FROM
(SELECT rownum rn FROM dual CONNECT BY rownum<=5) a, --5 menu items per rest.
(SELECT rownum rn FROM dual CONNECT BY rownum<=500000) b
);
commit;
Suppose you want to get the menu items
of the top 5 nearest restaurants to a particular location (10,0) for
an example. One way would be with the following spatial query (but
first of course collect the necessary stats):
BEGIN
DBMS_STATS.GATHER_TABLE_STATS(ownname=>user,tabname=>'restaurants',cascade=>TRUE);
DBMS_STATS.GATHER_TABLE_STATS(ownname=>user,tabname=>'menu_items',cascade=>TRUE, method_opt=>'FOR ALL INDEXED COLUMNS SIZE AUTO');
END;
SELECT t1.id
, t2.id
, t2.restaurants_id
, t2.item_number
FROM restaurants t1
, menu_items t2
WHERE t1.id=t2.restaurants_id
AND SDO_NN(t1.point,
SDO_GEOMETRY(
2001 -- 2 dimensional point
, null
, SDO_POINT_TYPE(10,0,null)
, null
, null
),
'sdo_num_res=5',
1
)='TRUE'
ORDER BY t1.id, t2.item_number;
The query produces the desired result.
However, let's look how it performed. I re-run the query, but this
time I used gather_plan_statistics hint to get the execution
statistics for the query.
SQL>SELECT /*+ gather_plan_statistics */ t1.id, t2.id, t2.restaurants_id, t2.item_number
2 FROM restaurants t1
3 , menu_items t2
4 WHERE t1.id=t2.restaurants_id
5 AND SDO_NN(t1.point,
6 SDO_GEOMETRY(
7 2001 -- 2 dimensional point
8 , null
9 , SDO_POINT_TYPE(10,0,null)
10 , null
11 , null
12 ),
13 'sdo_num_res=5',
14 1
15 )='TRUE'
16 ORDER BY t1.id, t2.item_number;
ID ID RESTAURANTS_ID ITEM_NUMBER
---------- ---------- -------------- -----------
8 8 8 1
8 500008 8 2
8 1000008 8 3
8 1500008 8 4
8 2000008 8 5
9 9 9 1
9 500009 9 2
9 1000009 9 3
9 1500009 9 4
9 2000009 9 5
10 10 10 1
10 500010 10 2
10 1000010 10 3
10 1500010 10 4
10 2000010 10 5
11 11 11 1
11 500011 11 2
11 1000011 11 3
11 1500011 11 4
11 2000011 11 5
12 12 12 1
12 500012 12 2
12 1000012 12 3
12 1500012 12 4
12 2000012 12 5
25 rows selected.
The query took 1.32 seconds to complete
as you can see from the execution statistics:
SQL> SELECT * FROM TABLE(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID gwpqub3k0awqm, child number 0
-------------------------------------
SELECT /*+ gather_plan_statistics */ t1.id, t2.id, t2.restaurants_id,
t2.item_number FROM restaurants t1 , menu_items t2 WHERE
t1.id=t2.restaurants_id AND SDO_NN(t1.point, SDO_GEOMETRY(
2001 -- 2 dimensional point , null ,
SDO_POINT_TYPE(10,0,null) , null , null ),
'sdo_num_res=5', 1 )='TRUE' ORDER BY t1.id, t2.item_number
Plan hash value: 2076547507
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 25 |00:00:01.32 | 41540 | 41465 | | | |
| 1 | SORT ORDER BY | | 1 | 24678 | 25 |00:00:01.32 | 41540 | 41465 | 2048 | 2048 | 2048 (0)|
|* 2 | HASH JOIN | | 1 | 24678 | 25 |00:00:01.32 | 41540 | 41465 | 1517K| 1517K| 886K (0)|
| 3 | TABLE ACCESS BY INDEX ROWID| RESTAURANTS | 1 | 5000 | 5 |00:00:00.01 | 64 | 0 | | | |
|* 4 | DOMAIN INDEX | RESTAURANTS_SIDX | 1 | | 5 |00:00:00.01 | 63 | 0 | | | |
| 5 | TABLE ACCESS FULL | MENU_ITEMS | 1 | 2500K| 2500K|00:00:00.62 | 41476 | 41465 | | | |
------------------------------------------------------------------------------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T1"."ID"="T2"."RESTAURANTS_ID")
4 - access("MDSYS"."SDO_NN"("T1"."POINT","MDSYS"."SDO_GEOMETRY"(2001,NULL,"SDO_POINT_TYPE"(10,0,NULL),NULL,NULL),'sdo_num_res=5',1)='
TRUE')
29 rows selected.
SQL>
The problem
Well, one could say that 1.32 seconds is
not that bad. However, if you look at the plan more carefully you can
notice that CBO used a HASH JOIN method to join RESTAURANTS and
MENU_ITEMS tables and as a result it performed full table scan on
MENU_ITEMS table. Now imagine if MENU_ITEMS table was quite big
(suppose you have stored the items of all restaurants across US)?!
What made CBO to pick a HASH JOIN?
Look at the estimated (5,000) vs. actual rows (5) selected from
RESTAURANTS table. We were only after the 5 nearest neighbors. We
used sdo_num_res=5 in our SDO_NN call. One could expect that CBO
would recognize that fact and estimate cardinality of 5. But, no, CBO applied selectivity of 1% (500,000
restaurants x 1% = 5,000)
Why selectivity of 1%?
SDO_NN is an operator that is bound to the implementation of the function NN that is part of PRVT_IDX package owned by MDSYS.
By default, statistics for PRVT_IDX package is defined by SDO_STATISTICS type. That is Extensible Optimizer feature is used to define how cardinality and the cost will be calculated for functions/procedures defined in the package. (if statistics are not associated, CBO uses default selectivity of 1%)
Seems like the logic implemented in ODCIStatsSelectivity() function is not good enough to detect that we are only after the first N rows, as defined with SDO_NUM_RES parameter.
Why selectivity of 1%?
SDO_NN is an operator that is bound to the implementation of the function NN that is part of PRVT_IDX package owned by MDSYS.
By default, statistics for PRVT_IDX package is defined by SDO_STATISTICS type. That is Extensible Optimizer feature is used to define how cardinality and the cost will be calculated for functions/procedures defined in the package. (if statistics are not associated, CBO uses default selectivity of 1%)
Seems like the logic implemented in ODCIStatsSelectivity() function is not good enough to detect that we are only after the first N rows, as defined with SDO_NUM_RES parameter.
You can clearly see this if you create
10053 trace file in the section where
MDSYS.SDO_STATISTICS.ODCIStatsSelectivity procedure is used to calculate selectivity.
MDSYS.SDO_STATISTICS.ODCIStatsFunctionCost returned:
CPUCost : 100000000000000
IOCost : 100000000000000
NetworkCost : -1
Calling user-defined selectivity function...
predicate: "MDSYS"."SDO_NN"("T1"."POINT","MDSYS"."SDO_GEOMETRY"(2001,NULL,"SDO_POINT_TYPE"(10,0,NULL),NULL,NULL),'sdo_num_res=5',1)='TRUE'
declare
sel number;
obj0 "MDSYS"."SDO_GEOMETRY" := "MDSYS"."SDO_GEOMETRY"(NULL, NULL, NULL, NULL, NULL);
begin
:1 := "MDSYS"."SDO_STATISTICS".ODCIStatsSelectivity(
sys.ODCIPREDINFO('MDSYS',
'PRVT_IDX',
'NN',
45),
sel,
sys.ODCIARGDESCLIST(sys.ODCIARGDESC(3, NULL, NULL, NULL, NULL, NULL, NULL), sys.ODCIARGDESC(3, NULL, NULL, NULL, NULL, NULL, NULL), sys.ODCIARGDESC(2, 'RESTAURANTS', 'TEST', '"POINT"', NULL, NULL, NULL), sys.ODCIARGDESC(3, NULL, NULL, NULL, NULL, NULL, NULL), sys.ODCIARGDESC(3, NULL, NULL, NULL, NULL, NULL, NULL)),
:3,
:4
, obj0, "MDSYS"."SDO_GEOMETRY"(2001,NULL,"SDO_POINT_TYPE"(10,0,NULL),NULL,NULL), :5,
sys.ODCIENV(:6,:7,:8,:9));
if sel IS NULL then
:2 := -1.0;
else
:2 := sel;
end if;
exception
when others then
raise;
end;
Bind :3 Value 'TRUE'
Bind :4 Value 'TRUE'
Bind :5 Value 'sdo_num_res=5'
ODCIEnv Bind :6 Value 0
ODCIEnv Bind :7 Value 0
ODCIEnv Bind :8 Value 0
ODCIEnv Bind :9 Value 3
MDSYS.SDO_STATISTICS.ODCIStatsSelectivity returned selectivity: 1.00000000%
Table: RESTAURANTS Alias: T1
Card: Original: 500000.000000 Rounded: 5000 Computed: 5000.00 Non Adjusted: 5000.00
If statistics are not associated CBO
would also use selectivity of 1%. So what is the differences? Under
what circumstances one could take advantage of the extensible
optimizer feature and generate better selectivity when SDO_NN operator is used along with sdo_num_res parameter? I couldn't find
any article or documentation piece that will help me answer these
questions.
What I do know however, is that it
shouldn't be that difficult to implement a piece of logic that will
make SDO_STATISTICS.ODCIStatsSelectivity procedure generate more
accurate numbers.
As a matter of fact, I played a bit
with this.
Disclaimer: Please be aware the
following examples are for demonstration purposes only and shouldn't
be used in an actual, supported, database environment. The logic I
show below is quite simple and is for illustration only. Please
contact Oracle Support if you want to implement something similar in
your environment.
I created a simple type named
SDO_STATISTICS_CUSTOM under MDSYS schema using the Extensible
Optimizer interface
(link
)
I implemented ODCIStatsSelectivity
procedure as shown below (one more time, this implementation is to
demonstrate that with appropriate logic ODCIStatsSelectivity can
return desired values):
STATIC FUNCTION ODCIStatsSelectivity (
pred_info IN SYS.ODCIPredInfo,
sel OUT NUMBER,
args IN SYS.ODCIArgDescList,
p_start IN VARCHAR2,
p_stop IN VARCHAR2,
geometry1 IN MDSYS.SDO_GEOMETRY,
geometry2 IN MDSYS.SDO_GEOMETRY,
param IN VARCHAR2,
env IN SYS.ODCIEnv
) RETURN NUMBER IS
BEGIN
*******************
if param=NULL then
sel:=1;
else if lower(substr(param,1,instr(param,'=')-1)) = 'sdo_num_res' then
SELECT decode(num_rows,null,1,
decode(to_number(trim(substr(param,instr(param,'=')+1))),0,1,to_number(trim(substr(param,instr(param,'=')+1)))/num_rows)*100
)
INTO sel
FROM dba_tables where owner=args(3).TABLESCHEMA and table_name=args(3).TABLENAME;
if sel>100 then
sel:=100 ;
end if;
end if;
end if;
****************
RETURN ODCIConst.success;
END ODCIStatsSelectivity;
The logic I chose is simple, if
sdo_num_res parameter is defined, get the number of records from the
statistics available for the table and set the selectivity variable
(sel) to (sdo_num_res/num_rows)*100.
Let's see how it works:
As you can see the cardinality is correctly calculated which resulted CBO to produce better execution plan. With this plan in place, the query completes in less than 1 cs.
If you look into 10053 trace file you can also see that the calculated selectivity is 0.001%:
Anyway, since the current implementation of SDO_STATISTICS type returns selectivity of 1%, the next logical question is what one can do in order to workaround the problem.
Let's see how it works:
SQL> EXPLAIN PLAN FOR
2 SELECT t1.id, t2.id, t2.restaurants_id, t2.item_number
3 FROM restaurants t1
4 , menu_items t2
5 WHERE t1.id=t2.restaurants_id
6 AND SDO_NN(t1.point,
7 SDO_GEOMETRY(
8 2001 -- 2 dimensional point
9 , null
10 , SDO_POINT_TYPE(10,0,null)
11 , null
12 , null
13 ),
14 'sdo_num_res=5',
15 1
16 )='TRUE'
17* ORDER BY t1.id, t2.item_number
SQL> /
Explained.
SQL> set line 200 pagesize 9999
SQL> SELECT * FROM table(DBMS_XPLAN.DISPLAY);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 2341889131
------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 25 | 1000 | 36 (3)| 00:00:01 |
| 1 | SORT ORDER BY | | 25 | 1000 | 36 (3)| 00:00:01 |
| 2 | NESTED LOOPS | | 25 | 1000 | 35 (0)| 00:00:01 |
| 3 | TABLE ACCESS BY INDEX ROWID| RESTAURANTS | 5 | 80 | 0 (0)| 00:00:01 |
|* 4 | DOMAIN INDEX | RESTAURANTS_SIDX | | | 0 (0)| 00:00:01 |
| 5 | TABLE ACCESS BY INDEX ROWID| MENU_ITEMS | 5 | 120 | 7 (0)| 00:00:01 |
|* 6 | INDEX RANGE SCAN | MENU_ITEMS_RESTAURANTS_IDX | 5 | | 2 (0)| 00:00:01 |
------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("MDSYS"."SDO_NN"("T1"."POINT","MDSYS"."SDO_GEOMETRY"(2001,NULL,"SDO_POINT_TYPE"(10,0,N
ULL),NULL,NULL),'sdo_num_res=5',1)='TRUE')
6 - access("T1"."ID"="T2"."RESTAURANTS_ID")
20 rows selected.
As you can see the cardinality is correctly calculated which resulted CBO to produce better execution plan. With this plan in place, the query completes in less than 1 cs.
If you look into 10053 trace file you can also see that the calculated selectivity is 0.001%:
MDSYS.SDO_STATISTICS_CUSTOM.ODCIStatsFunctionCost returned:
CPUCost : 10000
IOCost : 10000
NetworkCost : 10000
Calling user-defined selectivity function...
predicate: "MDSYS"."SDO_NN"("T1"."POINT","MDSYS"."SDO_GEOMETRY"(2001,NULL,"SDO_POINT_TYPE"(10,0,NULL),NULL,NULL),'sdo_num_res=5',1)='TRUE'
declare
sel number;
obj0 "MDSYS"."SDO_GEOMETRY" := "MDSYS"."SDO_GEOMETRY"(NULL, NULL, NULL, NULL, NULL);
begin
:1 := "MDSYS"."SDO_STATISTICS_CUSTOM".ODCIStatsSelectivity(
sys.ODCIPREDINFO('MDSYS',
'PRVT_IDX',
'NN',
45),
sel,
sys.ODCIARGDESCLIST(sys.ODCIARGDESC(3, NULL, NULL, NULL, NULL, NULL, NULL), sys.ODCIARGDESC(3, NULL, NULL, NULL, NULL, NULL, NULL), sys.ODCIARGDESC(2, 'RESTAURANTS', 'TEST', '"POINT"', NULL, NULL, NULL), sys.ODCIARGDESC(3, NULL, NULL, NULL, NULL, NULL, NULL), sys.ODCIARGDESC(3, NULL, NULL, NULL, NULL, NULL, NULL)),
:3,
:4
, obj0, "MDSYS"."SDO_GEOMETRY"(2001,NULL,"SDO_POINT_TYPE"(10,0,NULL),NULL,NULL), :5,
sys.ODCIENV(:6,:7,:8,:9));
if sel IS NULL then
:2 := -1.0;
else
:2 := sel;
end if;
exception
when others then
raise;
end;
Bind :3 Value 'TRUE'
Bind :4 Value 'TRUE'
Bind :5 Value 'sdo_num_res=5'
ODCIEnv Bind :6 Value 0
ODCIEnv Bind :7 Value 0
ODCIEnv Bind :8 Value 0
ODCIEnv Bind :9 Value 3
MDSYS.SDO_STATISTICS_CUSTOM.ODCIStatsSelectivity returned selectivity: 0.00100000%
Workaround
The problem can be addressed by
providing CBO that extra bit of information, that is how many
rows we expect to get back after applying SDO_NN operator. In order
to do this one could consider using the cardinality hint or rewrite
the query and add one more predicate ROWNUM<=5 that would tell CBO
how many rows we are expecting to select from RESTAURANTS table.
(there may be other strategies available too)
Use cardinality hint:
SELECT /*+ cardinality(t1 5) gather_plan_statistics */ t1.id, t2.id, t2.restaurants_id, t2.item_number
FROM restaurants t1
, menu_items t2
WHERE t1.id=t2.restaurants_id
AND SDO_NN(t1.point,
SDO_GEOMETRY(
2001 -- 2 dimensional point
, null
, SDO_POINT_TYPE(10,0,null)
, null
, null
),
'sdo_num_res=5',
1
)='TRUE'
ORDER BY t1.id, t2.item_number;
The execution statistics in this case
look like:
Plan hash value: 2341889131
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 25 |00:00:00.01 | 92 | 5 | | | |
| 1 | SORT ORDER BY | | 1 | 25 | 25 |00:00:00.01 | 92 | 5 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS | | 1 | 25 | 25 |00:00:00.01 | 92 | 5 | | | |
| 3 | TABLE ACCESS BY INDEX ROWID| RESTAURANTS | 1 | 5 | 5 |00:00:00.01 | 55 | 0 | | | |
|* 4 | DOMAIN INDEX | RESTAURANTS_SIDX | 1 | | 5 |00:00:00.01 | 54 | 0 | | | |
| 5 | TABLE ACCESS BY INDEX ROWID| MENU_ITEMS | 5 | 5 | 25 |00:00:00.01 | 37 | 5 | | | |
|* 6 | INDEX RANGE SCAN | MENU_ITEMS_RESTAURANTS_IDX | 5 | 5 | 25 |00:00:00.01 | 12 | 0 | | | |
----------------------------------------------------------------------------------------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("MDSYS"."SDO_NN"("T1"."POINT","MDSYS"."SDO_GEOMETRY"(2001,NULL,"SDO_POINT_TYPE"(10,0,NULL),NULL,NULL),'sdo_num_res=5',1)='TRUE')
6 - access("T1"."ID"="T2"."RESTAURANTS_ID")
29 rows selected.
Rewrite the query and use ROWNUM<=n
predicate where n is the number associated with sdo_num_rows
parameter:
SELECT /*+ gather_plan_statistics */ t1.id, t2.id, t2.restaurants_id, t2.item_number
FROM (SELECT *
FROM restaurants
WHERE
SDO_NN(point,
SDO_GEOMETRY(
2001 -- 2 dimensional point
, null
, SDO_POINT_TYPE(10,0,null)
, null
, null
),
'sdo_num_res=5',
1
)='TRUE'
AND rownum<=5
) t1
, menu_items t2
WHERE t1.id=t2.restaurants_id
ORDER BY t1.id, t2.item_number;
… and the execution statistics :
Plan hash value: 3570932640
----------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 25 |00:00:00.01 | 101 | | | |
| 1 | SORT ORDER BY | | 1 | 25 | 25 |00:00:00.01 | 101 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS | | 1 | | 25 |00:00:00.01 | 101 | | | |
| 3 | NESTED LOOPS | | 1 | 25 | 25 |00:00:00.01 | 76 | | | |
| 4 | VIEW | | 1 | 5 | 5 |00:00:00.01 | 64 | | | |
|* 5 | COUNT STOPKEY | | 1 | | 5 |00:00:00.01 | 64 | | | |
| 6 | TABLE ACCESS BY INDEX ROWID| RESTAURANTS | 1 | 5 | 5 |00:00:00.01 | 64 | | | |
|* 7 | DOMAIN INDEX | RESTAURANTS_SIDX | 1 | | 5 |00:00:00.01 | 63 | | | |
|* 8 | INDEX RANGE SCAN | MENU_ITEMS_RESTAURANTS_IDX | 5 | 5 | 25 |00:00:00.01 | 12 | | | |
| 9 | TABLE ACCESS BY INDEX ROWID | MENU_ITEMS | 25 | 5 | 25 |00:00:00.01 | 25 | | | |
----------------------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - filter(ROWNUM<=5)
7 - access("MDSYS"."SDO_NN"("POINT","MDSYS"."SDO_GEOMETRY"(2001,NULL,"SDO_POINT_TYPE"(10,0,NULL),NULL,NULL),'sdo_num_res=5',1)='TRUE')
8 - access("T1"."ID"="T2"."RESTAURANTS_ID")
34 rows selected.
As you can see, Oracle CBO made correct cardinality estimates in the last two examples and used NESTED LOOPS method to
join both tables that resulted with performance improvement.
I hope you got some valuable information out of this post.
