Question

I’m trying to create a subset of a table (as a materialized view), defined as those records which have a matching record in another materialized view.

For example, let’s say I have a Users table with user_id and name columns, and a Log table, with entry_id, user_id, activity, and timestamp columns.

First I create a materialized view of the Log table, selecting only those rows with timestamp > some_date. Now I want a materliazed view of the Users referenced in my snapshot of the Log table. I can either create it as

select * from Users where user_id in (select user_id from Log_mview)

or I can do

select distinct u.* from Users u inner join Log_mview l on u.user_id = l.user_id

(need the distinct to avoid multiple hits from users with multiple log entries).

The former seems cleaner and more elegant, but takes much longer. Am I missing something? Is there a better way to do this?

Edit: The where exists clause helped a lot, except in the case where the condition uses an OR. For example, let’s say the Log table above also had a user_name column, and the correct way to match a Log entry to a Users record is when either of the columns (user id or user name) match. I’m finding that

select distinct u.* from Users u
    inner join Log_mview l
        on u.user_id = l.user_id or u.name = l.user_name

is much faster than

select * from Users u where exists
    (select id from Log_mview l 
        where l.user_id = u.user_id or l.user_name = u.name)

Any help?

(Regarding the explain plan… Lemme work on sanitizing it, or them, rather… I’ll post them in a while.)

Edit: explain plans:
For the query with inner join:

Plan hash value: 436698422

---------------------------------------------------------------------------------------------------------------
| Id  | Operation                       | Name                | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                |                     |  4539K|   606M|       |   637K  (3)| 02:07:25 |
|   1 |  HASH UNIQUE                    |                     |  4539K|   606M|  3201M|   637K  (3)| 02:07:25 |
|   2 |   CONCATENATION                 |                     |       |       |       |            |          |
|*  3 |    HASH JOIN                    |                     |  4206K|   561M|    33M|   181K  (4)| 00:36:14 |
|   4 |     BITMAP CONVERSION TO ROWIDS |                     |   926K|    22M|       |  2279   (1)| 00:00:28 |
|   5 |      BITMAP INDEX FAST FULL SCAN| I_M_LOG_MVIEW_4     |       |       |       |            |          |
|*  6 |     TABLE ACCESS FULL           | USERS               |    15M|  1630M|       | 86638   (6)| 00:17:20 |
|*  7 |    HASH JOIN                    |                     |  7646K|  1020M|    33M|   231K  (4)| 00:46:13 |
|   8 |     BITMAP CONVERSION TO ROWIDS |                     |   926K|    22M|       |  2279   (1)| 00:00:28 |
|   9 |      BITMAP INDEX FAST FULL SCAN| I_M_LOG_MVIEW_4     |       |       |       |            |          |
|  10 |     TABLE ACCESS FULL           | USERS               |    23M|  2515M|       | 87546   (7)| 00:17:31 |
---------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("U"."NAME"="L"."USER_NAME")
   6 - filter("U"."NAME" IS NOT NULL)
   7 - access("U"."USER_ID"=TO_NUMBER("L"."USER_ID"))
       filter(LNNVL("U"."NAME"="L"."USER_NAME") OR LNNVL("U"."NAME" IS NOT NULL))

Note
-----
   - dynamic sampling used for this statement

For the one using where exists:

Plan hash value: 2786958565

-----------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                     |     1 |   114 |    21M  (1)| 70:12:13 |
|*  1 |  FILTER                       |                     |       |       |            |          |
|   2 |   TABLE ACCESS FULL           | USERS               |    23M|  2515M| 87681   (7)| 00:17:33 |
|   3 |   BITMAP CONVERSION TO ROWIDS |                     |  7062 |   179K|     1   (0)| 00:00:01 |
|*  4 |    BITMAP INDEX FAST FULL SCAN| I_M_LOG_MVIEW_4     |       |       |            |          |
-----------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter( EXISTS (SELECT /*+ */ 0 FROM "MYSCHEMA"."LOG_MVIEW" 
              "LOG_MVIEW" WHERE ("USER_NAME"=:B1 OR TO_NUMBER("USER_ID")=:B2) AND 
              ("USER_NAME"=:B3 OR TO_NUMBER("USER_ID")=:B4) AND ("USER_NAME"=:B5 OR 
              TO_NUMBER("USER_ID")=:B6)))
   4 - filter("USER_NAME"=:B1 OR TO_NUMBER("USER_ID")=:B2)

Note
-----
   - dynamic sampling used for this statement

DB object names changed to protect the innocent. :p

Answer 1

Try this

select * from Users u
where exists 
   ( select user_id 
     from Log_mview l
     where l.user_id = u.user_id )
/

If the sub-query returns a large number of rows WHERE EXISTS can be substantially faster than WHERE ... IN.