Can this query be made faster? - sql

Question

My Oracle query takes over 1.5 min and I do not know if it's because of inefficient query writing, bad choice of indexes or some other database issue that I cannot control.
Some tables and data were changed to protect IP.
SELECT /*+ PARALLEL (AUTO) */ COUNT(DISTINCT SUD_USERID)
FROM (
SELECT /*+ PARALLEL (AUTO) */
SUD_USERID ,
CASE WHEN SCH_PAGETYPE = 'Page' AND SUD_EVENTTYPE = 'S'
THEN 'EVENTTYPE1'
WHEN SCH_PAGETYPE = 'Page' AND SUD_EVENTTYPE = 'V'
THEN 'EVENTTYPE2'
WHEN SCH_PAGETYPE = 'Hub' AND SUD_EVENTTYPE = 'S'
THEN 'EVENTTYPE3'
END AS CALC_EVENT_SOURCE,
SUD_EVENT_SOURCE
FROM
(
SELECT /*+ PARALLEL (AUTO) */
UPPER(PAGETYPE)|| '-' || SCH.ID PAGETYPE_ID ,
SCH.PAGETYPE SCH_PAGETYPE
FROM TABLE1 SCH
WHERE SCH.PAGETYPE IN ('Page', 'Hub')
AND SCH.CATEGORY_NAME NOT IN ('archive', 'testcategory')
)
INNER JOIN (
SELECT /*+ PARALLEL (AUTO) */
DISTINCT SUD.TRACEID TRACEID ,
SUD.EVENTTYPE SUD_EVENTTYPE ,
SUD.USERID SUD_USERID,
SUD.EVENT_SOURCE SUD_EVENT_SOURCE
FROM
SOMESCHEMA.USAGE_DETAILS SUD
WHERE
SUD.EVENTTYPE IN ('S', 'V')
)
ON TRACEID = PAGETYPE_ID
INNER JOIN USER_JOB_FAMILY_MAPPING SFD
ON SUD_USERID = SFD.USERID
)
WHERE CALC_EVENT_SOURCE = SUD_EVENT_SOURCE
I could not copy the text of the explain plan (generated via DBeaver)
but here is a screenshot:
USAGE_DETAILS table has 3941810 rows
TABLE1 has 5908 rows
USER_JOB_FAMILY_MAPPING has 578233 rows
There are no keys on any of these tables.
USAGE_DETAILS.TRACEID is VARCHAR2(500) NOT NULL has function index=SUBSTR("TRACEID",1,4) and
another index declared as default but on that column.
USAGE_DETAILS.USERID is VARCHAR2(50) NOT NULL
USAGE_DETAILS.EVENTTYPE is VARCHAR2(10) NOT NULL and has default index
USAGE_DETAILS.EVENT_SOURCE is VARCHAR2(200) NOT NULL and has default index
I have tried doing inner joins on the full tables rather than the parenthetically generated (subselect?) tables, but that did not perform better and also limited my ability to use an alias in the WHERE clause.
I do not know what kind of machine this is running on, just that it's set up for development. I'd like this query to give me accurate answers in under 10s. Some times the query above though still does not return even after 10+ minutes.
Plan hash value: 2784166315
-----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 27 | | 258K (1)| 00:00:11 |
| 1 | SORT AGGREGATE | | 1 | 27 | | | |
| 2 | VIEW | VM_NWVW_1 | 1809K| 46M| | 258K (1)| 00:00:11 |
| 3 | HASH GROUP BY | | 1809K| 745M| | 258K (1)| 00:00:11 |
|* 4 | HASH JOIN | | 1809K| 745M| | 258K (1)| 00:00:11 |
|* 5 | TABLE ACCESS FULL | TABLE1S | 5875 | 172K| | 309 (0)| 00:00:01 |
| 6 | MERGE JOIN SEMI | | 3079K| 1180M| | 257K (1)| 00:00:11 |
| 7 | SORT JOIN | | 3079K| 1139M| | 254K (1)| 00:00:10 |
| 8 | VIEW | | 3079K| 1139M| | 254K (1)| 00:00:10 |
| 9 | HASH UNIQUE | | 3079K| 1139M| 1202M| 254K (1)| 00:00:10 |
| 10 | INLIST ITERATOR | | | | | | |
| 11 | TABLE ACCESS BY INDEX ROWID BATCHED| USAGE_DETAILS | 3079K| 1139M| | 46 (0)| 00:00:01 |
|* 12 | INDEX RANGE SCAN | IDX_UUD_EVENTTYPE | 13704 | | | 46 (0)| 00:00:01 |
|* 13 | SORT UNIQUE | | 578K| 7905K| 22M| 3558 (1)| 00:00:01 |
| 14 | INDEX FAST FULL SCAN | USERID_IDX | 578K| 7905K| | 704 (1)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("TRACEID"=UPPER("PAGETYPE")||'-'||TO_CHAR("SCH"."ID"))
filter("from$_subquery$_004"."SUD_EVENT_SOURCE"=CASE WHEN (("SCH"."PAGETYPE"='Page') AND
("from$_subquery$_004"."SUD_EVENTTYPE"='S')) THEN 'EVENTTYPE1' WHEN (("SCH"."PAGETYPE"='Page') AND
("from$_subquery$_004"."SUD_EVENTTYPE"='V')) THEN 'EVENTTYPE2' WHEN (("SCH"."PAGETYPE"='Hub') AND
("from$_subquery$_004"."SUD_EVENTTYPE"='S')) THEN 'EVENTTYPE3' END )
5 - filter("SCH"."CATEGORY_NAME"<>'archive' AND "SCH"."CATEGORY_NAME"<>'testcategory' AND ("SCH"."PAGETYPE"='Hub' OR
"SCH"."PAGETYPE"='Page'))
12 - access("SUD"."EVENTTYPE"='S' OR "SUD"."EVENTTYPE"='V')
13 - access("from$_subquery$_004"."SUD_USERID"="SFD"."USERID")
filter("from$_subquery$_004"."SUD_USERID"="SFD"."USERID")
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
- automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation
After running
exec dbms_stats.gather_table_stats(ownname=>'SCHEMA1',tabname=>'USAGE_DETAILS');
exec dbms_stats.gather_table_stats(ownname=>'SCHEMA1',tabname=>'TABLE1');
I have this new plan:
SQL> select plan_table_output from table(dbms_xplan.display());
Plan hash value: 3419946982
----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 27 | | 70152 (1)| 00:00:03 |
| 1 | SORT AGGREGATE | | 1 | 27 | | | |
| 2 | VIEW | VM_NWVW_1 | 53144 | 1401K| | 70152 (1)| 00:00:03 |
| 3 | HASH GROUP BY | | 53144 | 21M| 21M| 70152 (1)| 00:00:03 |
|* 4 | HASH JOIN RIGHT SEMI | | 53144 | 21M| 14M| 65453 (1)| 00:00:03 |
| 5 | INDEX FAST FULL SCAN | USERID_IDX | 578K| 7905K| | 704 (1)| 00:00:01 |
|* 6 | HASH JOIN | | 53144 | 20M| | 62995 (1)| 00:00:03 |
| 7 | JOIN FILTER CREATE | :BF0000 | 5503 | 161K| | 309 (0)| 00:00:01 |
|* 8 | TABLE ACCESS FULL | TABLE1 | 5503 | 161K| | 309 (0)| 00:00:01 |
| 9 | VIEW | | 3549K| 1259M| | 62677 (1)| 00:00:03 |
| 10 | HASH UNIQUE | | 3549K| 159M| 203M| 62677 (1)| 00:00:03 |
| 11 | JOIN FILTER USE | :BF0000 | 3549K| 159M| | 21035 (1)| 00:00:01 |
|* 12 | TABLE ACCESS FULL| USAGE_DETAILS | 3549K| 159M| | 21035 (1)| 00:00:01 |
----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("from$_subquery$_004"."SUD_USERID"="SFD"."USERID")
6 - access("TRACEID"=UPPER("PAGETYPE")||'-'||TO_CHAR("SCH"."ID"))
filter("from$_subquery$_004"."SUD_EVENT_SOURCE"=CASE WHEN (("SCH"."PAGETYPE"='Page') AND
("from$_subquery$_004"."SUD_EVENTTYPE"='S')) THEN 'EVENTTYPE1' WHEN (("SCH"."PAGETYPE"='Page') AND
("from$_subquery$_004"."SUD_EVENTTYPE"='V')) THEN 'EVENTTYPE2' WHEN (("SCH"."PAGETYPE"='Hub') AND
("from$_subquery$_004"."SUD_EVENTTYPE"='S')) THEN 'EVENTTYPE3' END )
8 - filter("SCH"."CATEGORY_NAME"<>'archive' AND "SCH"."CATEGORY_NAME"<>'testcategory' AND
("SCH"."PAGETYPE"='Hub' OR "SCH"."PAGETYPE"='Page'))
12 - filter(("SUD"."EVENTTYPE"='S' OR "SUD"."EVENTTYPE"='V') AND
SYS_OP_BLOOM_FILTER(:BF0000,"SUD"."TRACEID"))
Note
-----
- automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation
37 rows selected.
Is it more likely that the compute statistics massively helped this query or that someone did something else that I was not aware of? Yes, the query ran much better, but I'd feel better too if I knew why.

Answer 1

Hy,
after reviewing your SQL I noticed your statements are full of string comparisons and searches. For example
SELECT /*+ PARALLEL (AUTO) */
UPPER(PAGETYPE)|| '-' || SCH.ID PAGETYPE_ID ,
SCH.PAGETYPE SCH_PAGETYPE
FROM TABLE1 SCH
WHERE SCH.PAGETYPE IN ('Page', 'Hub')
AND SCH.CATEGORY_NAME NOT IN ('archive', 'testcategory')
This can be indexed int 2 ways.
First: Create table that has 'Page', 'Hub', and other types that you need, create for the a column Index and then "replace" basically adapt your query to resolve those indexes instead of string compare.
Tables can have multiple indices on columns those have to be treated with caution because they create problems in regards of database size.
Also I would check if what are the biggest tables and reorder their selections to the last. Meaning:
if one table has 12 rows and the other 100. First put the 12 row table then the 100 row. This will multiply in your case since the tables and nested and chained.
I made 1 more review and realized I made an oversight.
USAGE_DETAILS table has 3941810 rows
TABLE1 has 5908 rows
USER_JOB_FAMILY_MAPPING has 578233 rows
First filter the table 1, this is costly already, then Inner join raw USAGE_DETAILS and then select the join ID-s.
Then inner join USER_JOB_FAMILY_MAPPING, and select after that. The reason is that the joins are done on the ID which is probably int type.

Answer 2

Gather statistics on the relevant objects like this:
begin
dbms_stats.gather_table_stats(ownname => user, tabname => 'TABLE1');
dbms_stats.gather_table_stats(ownname => 'SOMESCHEMA', tabname => 'USAGE_DETAILS');
end;
/
This line in the execution plan implies that one of the tables is missing statistics:
- dynamic statistics used: dynamic sampling (level=2)
Not all uses of dynamic sampling imply missing statistics, but level 2 is highly suspicious. That sampling level is usually intended to "Apply dynamic sampling to all unanalyzed tables."
Optimizer statistics are necessary for Oracle to make good execution plans. The algorithms and access paths for joining small amounts of data are different than the algorithms and access paths for joining large amounts of data. The optimizer statistics help Oracle estimate the size of the results and build good plans.
If this solves your problem, you should also investigate the root cause. Optimizer statistics should always be gathered manually after a large change, and automatically by the system every night. If you have a large ETL process that significantly changes a table, it should include a call to DBMS_STATS at the end. The database by default gathers stats at 10PM every night, unless a DBA foolishly disabled the autotask.
If that doesn't solve the problem, then regenerate the execution plan with actual numbers using DBMS_SQLTUNE or the GATHER_PLAN_STATISTICS_HINT. SQL tuning is about optimizing the operations. Your SQL statement has 14 operations, each of which is like a miniature program. We need to know which one of the operations is causing the problem. Finding actual cardinalities and actual run times, and comparing them to estimates, helps tremendously with diagnosing SQL problems.
How do we know that gathering stats was what fixed the performance?
We can't be 100% sure. But it's a safe bet that gathering statistics was responsible for the improvement, for several reasons.
Bad or missing statistics are responsible for a large percentage of all Oracle performance problems. Ask any DBA and they'll have plenty of stories about missing statistics.
The Note section changes strongly imply there are no other weird things happening behind the scenes. There are lots of tricks to silently fix queries, like SQL profiles, baselines, adaptive reoptimization, dynamic sampling (shows up in the first plan, but not the second one, because stats are better), etc. But if those tricks were used they would show up in the Note section.