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RE: Why the index is not used ?

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Hi Pavel

 

               Thanks you for your answer. here is a procedure that works :

-        CREATE TABLE cartedecredit(card_id SERIAL PRIMARY KEY, username VARCHAR(100), cc bytea);

-        INSERT INTO cartedecredit(username,cc)  SELECT 'individu ' || x.id, pgp_sym_encrypt('test value ' || x.id, 'motdepasse','compress-algo=2, cipher-algo=aes256') FROM generate_series(1,100000) AS x(id);  

-        CREATE INDEX idx_cartedecredit_cc02 ON cartedecredit(pgp_sym_decrypt(cc, 'motdepasse','compress-algo=2, cipher-algo=aes256'));

-        SELECT pgp_sym_decrypt(cc, 'motdepasse') FROM cartedecredit WHERE pgp_sym_decrypt(cc, 'motdepasse','compress-algo=2, cipher-algo=aes256')='test value 32';

pgp_sym_decrypt

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

test value 32

(1 row)

 

Time: 2.237 ms

-         explain analyze SELECT pgp_sym_decrypt(cc, 'motdepasse') FROM cartedecredit WHERE pgp_sym_decrypt(cc, 'motdepasse','compress-algo=2, cipher-algo=aes256')='test value 32';

                                                              QUERY PLAN

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

Index Scan using idx_cartedecredit_cc02 on cartedecredit  (cost=0.42..8.44 rows=1 width=32) (actual time=1.545..1.546 rows=1 loops=1)

   Index Cond: (pgp_sym_decrypt(cc, 'motdepasse'::text, 'compress-algo=2, cipher-algo=aes256'::text) = 'test value 32'::text)

Planning time: 0.330 ms

Execution time: 1.580 ms

(4 rows)

 

OK that works great.

Thank you for the recommendation

 

Best Regards

 

cid:image002.png@01D14E0E.8515EB90


Didier ROS

Expertise SGBD

DS IT/IT DMA/Solutions Groupe EDF/Expertise Applicative - SGBD
Nanterre Picasso - E2 565D (aile nord-est)
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92000 Nanterre

didier.ros@xxxxxx
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Tél. : 01 78 66 61 14
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Lync : ros.didier@xxxxxx

 

 

De : pavel.stehule@xxxxxxxxx [mailto:pavel.stehule@xxxxxxxxx]
Envoyé : samedi 6 octobre 2018 12:14
À : ROS Didier <didier.ros@xxxxxx>
Cc : pgsql-sql@xxxxxxxxxxxxxxxxxxxx; pgsql-performance@xxxxxxxxxxxxxxxxxxxx; pgsql-general@xxxxxxxxxxxxxxxxxxxx
Objet : Re: Why the index is not used ?

 

 

so 6. 10. 2018 v 11:57 odesílatel ROS Didier <didier.ros@xxxxxx> napsal:

Hi

I would like to submit the following problem to the PostgreSQL community. In my company, we have data encryption needs.
So I decided to use the following procedure :

 

(1)    Creating a table with a bytea type column to store the encrypted data
CREATE TABLE cartedecredit(card_id SERIAL PRIMARY KEY, username VARCHAR(100), cc bytea);

 

(2)    inserting encrypted data
INSERT INTO cartedecredit(username,cc)  SELECT 'individu ' || x.id, pgp_sym_encrypt('test value ' || x.id, 'motdepasse','compress-algo=2, cipher-algo=aes256') FROM generate_series(1,100000) AS x(id);  

 

(3)    Querying the table
SELECT pgp_sym_decrypt(cc, 'motdepasse') FROM cartedecredit WHERE pgp_sym_decrypt(cc, 'motdepasse')='test value 32';

pgp_sym_decrypt

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

test value 32

(1 row)

 

Time: 115735.035 ms (01:55.735)
-> the execution time is very long. So, I decide to create an index

 

(4)    Creating an index on encrypted data
CREATE INDEX idx_cartedecredit_cc02 ON cartedecredit(cc);

 

this index cannot to help.

 

but functional index can cartedecredit(pgp_sym_decrypt(cc, 'motdepasse'). Unfortunately index file will be decrypted in this case.

 

CREATE INDEX ON

 

 

(5)    Querying the table again

SELECT pgp_sym_decrypt(cc, 'motdepasse') FROM cartedecredit WHERE pgp_sym_decrypt(cc, 'motdepasse')='test value 32';
pgp_sym_decrypt

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

test value 32

(1 row)

 

Time: 118558.485 ms (01:58.558) -> almost 2 minutes !!
postgres=# explain analyze SELECT pgp_sym_decrypt(cc, 'motdepasse') FROM cartedecredit WHERE pgp_sym_decrypt(cc, 'motdepasse')='test value 32';

                                                      QUERY PLAN

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

Seq Scan on cartedecredit  (cost=0.00..3647.25 rows=500 width=32) (actual time=60711.787..102920.509 rows=1 loops=1)

   Filter: (pgp_sym_decrypt(cc, 'motdepasse'::text) = 'test value 32'::text)

   Rows Removed by Filter: 99999

Planning time: 0.112 ms

Execution time: 102920.585 ms

(5 rows)

 

è the index is not used in the execution plan. maybe because of the use of a function in the WHERE clause. I decide to modify the SQL query

 

(6)    Querying the table
SELECT pgp_sym_decrypt(cc, 'motdepasse') FROM cartedecredit WHERE cc=pgp_sym_encrypt('test value 32', 'motdepasse');

 

it is strange - this should to use index, when there is usual index over cc column.

 

What is result of explain analyze when you penalize seq scan by

 

set enable_seqscan to off

 

 

pgp_sym_decrypt

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

(0 rows)

 

Time: 52659.571 ms (00:52.660)

è The execution time is very long and I get no result (!?)

                                                    QUERY PLAN

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

Seq Scan on cartedecredit  (cost=0.00..3646.00 rows=1 width=32) (actual time=61219.989..61219.989 rows=0 loops=1)

   Filter: (cc = pgp_sym_encrypt('test value 32'::text, 'motdepasse'::text))

   Rows Removed by Filter: 100000

Planning time: 0.157 ms

Execution time: 61220.035 ms

(5 rows)

 

è My index is not used.


QUESTIONS :
-      why I get no result ?

-        why the index is not used?

Thanks in advance

 

Best Regards
Didier

 

 

cid:image002.png@01D14E0E.8515EB90


Didier ROS

Expertise SGBD

DS IT/IT DMA/Solutions Groupe EDF/Expertise Applicative - SGBD

 

 


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