Re: Why the index is not used ?

so 6. 10. 2018 v 11:57 odesílatel ROS Didier <[email protected]> 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
>
>
>
>
>
> [image: cid:[email protected]]
>
>
> * Didier ROS*
> * Expertise SGBD*
>
>
> *DS IT/IT DMA/Solutions Groupe EDF/Expertise Applicative - SGBD *
>
>
>
>
>
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