Query Regarding Explain Plan on Query

Question

Hello,
I have one big query which shows report of 50000 daily records from @ 20,00,000 records.
I have two databases UAT and PROD.when i do Explain Plan on the query is these different database i get the different plan where everything is same in both database.
In UAT it is doing Index scan where as in PROD it is doing Full TableScan. Below are the results.
In production it is not using any of the indexes present but in UAT it is.What could be the reasong behind this?

Answer 1

Sure.
UAT Explain Plan (Please copy in Textpad for better View)
SELECT STATEMENT, GOAL = HINT: ALL_ROWS     Cost=371     Cardinality=238     Optimizer=HINT: ALL_ROWS     Bytes=134470
VIEW     Object owner=SWNET1          Cost=371     Cardinality=238          Bytes=134470
COUNT STOPKEY                              
VIEW     Object owner=SWNET1          Cost=371     Cardinality=238          Bytes=131376
SORT ORDER BY STOPKEY          Cost=371     Cardinality=238          Bytes=54026
FILTER                              
HASH JOIN RIGHT ANTI          Cost=370     Cardinality=238          Bytes=54026
INLIST ITERATOR                              
TABLE ACCESS BY INDEX ROWID     Object owner=SWNET1     Object name=IS_TB_END_POINT     Cost=1     Cardinality=1     Optimizer=ANALYZED     Bytes=31
INDEX RANGE SCAN          Object owner=SWNET1     Object name=IS_UK_EP_NAME     Cost=1     Cardinality=1     Optimizer=ANALYZED     
TABLE ACCESS BY INDEX ROWID     Object owner=SWNET1     Object name=IS_TB_TRANSACTION     Cost=368     Cardinality=253     Optimizer=ANALYZED     Bytes=49588
INDEX FULL SCAN          Object owner=SWNET1     Object name=IS_IX_T_DESTINATION_EP     Cost=18     Cardinality=13909     Optimizer=ANALYZED
PRODUCTION Explain Plan
SELECT STATEMENT, GOAL = HINT: ALL_ROWS               Cost=65702     Cardinality=1000     Optimizer=HINT: ALL_ROWS     Bytes=565000
VIEW     Object owner=SWNET1          Cost=65702     Cardinality=1000          Bytes=565000
COUNT STOPKEY                              
VIEW     Object owner=SWNET1          Cost=65702     Cardinality=38739          Bytes=21383928
SORT ORDER BY STOPKEY          Cost=65702     Cardinality=38739          Bytes=9646011
FILTER                              
HASH JOIN RIGHT ANTI          Cost=63616     Cardinality=38739          Bytes=9646011
INLIST ITERATOR                              
TABLE ACCESS BY INDEX ROWID     Object owner=SWNET1     Object name=IS_TB_END_POINT     Cost=1     Cardinality=2     Optimizer=ANALYZED     Bytes=64
INDEX UNIQUE SCAN          Object owner=SWNET1     Object name=IS_UK_EP_NAME     Cost=1     Cardinality=2     Optimizer=ANALYZED     
TABLE ACCESS FULL          Object owner=SWNET1     Object name=IS_TB_TRANSACTION     Cost=63614     Cardinality=44697     Optimizer=ANALYZED     Bytes=9699249
Index Query (Same on both places)
create index IS_IX_T_DESTINATION_EP on IS_TB_TRANSACTION (T_DESTINATION_EP)
tablespace IS_XML_IND
pctfree 10
initrans 2
maxtrans 255
storage
initial 128M
next 128K
minextents 1
maxextents unlimited
pctincrease 0
);