RE: Why the index is not used ?

[ROS Didier <didier.ros@xxxxxx>]

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

 

	Didier ROS Expertise SGBD DS IT/IT DMA/Solutions Groupe EDF/Expertise Applicative - SGBD Nanterre Picasso - E2 565D (aile nord-est) 32 Avenue Pablo Picasso 92000 Nanterre didier.ros@xxxxxx support-postgres-niveau3@xxxxxx support-oracle-niveau3@xxxxxx Tél. : 01 78 66 61 14 Tél. mobile : 06 49 51 11 88 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

 

 

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

 

 

Ce message et toutes les pièces jointes (ci-après le 'Message') sont établis à l'intention exclusive des destinataires et les informations qui y figurent sont strictement confidentielles. Toute utilisation de ce Message non conforme à sa destination, toute diffusion ou toute publication totale ou partielle, est interdite sauf autorisation expresse.

Si vous n'êtes pas le destinataire de ce Message, il vous est interdit de le copier, de le faire suivre, de le divulguer ou d'en utiliser tout ou partie. Si vous avez reçu ce Message par erreur, merci de le supprimer de votre système, ainsi que toutes ses copies, et de n'en garder aucune trace sur quelque support que ce soit. Nous vous remercions également d'en avertir immédiatement l'expéditeur par retour du message.

Il est impossible de garantir que les communications par messagerie électronique arrivent en temps utile, sont sécurisées ou dénuées de toute erreur ou virus.
____________________________________________________

This message and any attachments (the 'Message') are intended solely for the addressees. The information contained in this Message is confidential. Any use of information contained in this Message not in accord with its purpose, any dissemination or disclosure, either whole or partial, is prohibited except formal approval.

If you are not the addressee, you may not copy, forward, disclose or use any part of it. If you have received this message in error, please delete it and all copies from your system and notify the sender immediately by return message.

E-mail communication cannot be guaranteed to be timely secure, error or virus-free.

Ce message et toutes les pièces jointes (ci-après le 'Message') sont établis à l'intention exclusive des destinataires et les informations qui y figurent sont strictement confidentielles. Toute utilisation de ce Message non conforme à sa destination, toute diffusion ou toute publication totale ou partielle, est interdite sauf autorisation expresse.

Si vous n'êtes pas le destinataire de ce Message, il vous est interdit de le copier, de le faire suivre, de le divulguer ou d'en utiliser tout ou partie. Si vous avez reçu ce Message par erreur, merci de le supprimer de votre système, ainsi que toutes ses copies, et de n'en garder aucune trace sur quelque support que ce soit. Nous vous remercions également d'en avertir immédiatement l'expéditeur par retour du message.

Il est impossible de garantir que les communications par messagerie électronique arrivent en temps utile, sont sécurisées ou dénuées de toute erreur ou virus.
____________________________________________________

This message and any attachments (the 'Message') are intended solely for the addressees. The information contained in this Message is confidential. Any use of information contained in this Message not in accord with its purpose, any dissemination or disclosure, either whole or partial, is prohibited except formal approval.

If you are not the addressee, you may not copy, forward, disclose or use any part of it. If you have received this message in error, please delete it and all copies from your system and notify the sender immediately by return message.

E-mail communication cannot be guaranteed to be timely secure, error or virus-free.