Re: Tuning Self-referencing Inserts
- From: Andy Sayer <andysayer@xxxxxxxxx>
- To: Tim Gorman <tim.evdbt@xxxxxxxxx>
- Date: Sun, 2 Dec 2018 23:15:00 +0000
The table is very small about 64K.
What's more important is how much of the table is being read and how often.
Here is an explain plan from a month ago.
An explain plan from a month ago could be very misleading, you seem to have
access to the relevant privileges so you should be looking at the real
execution plan in memory using
select * from table(dbms_xplan.display_cursor(sql_id=><the sql_id>));
The actual explain plan tells us the query is more complicated than a
simple select * from table.
This should be taken with a pinch of salt as it's an explain plan (so
unreliable) from a month ago (so the stats would have changed)
We can see that most of the cost in the query is in the index range scan on
line 10, it's not often you see an index range scan cost over 1,000 and it
usually suggests you are using a filter with high index selectivity
(reading lots of the index) - this is quite a frequent mistake I see, just
because an index is being used, it doesn't mean it's being used well. The
predicates section of the plan will tell you exactly what's going on, you
should always include it (and the notes) when sharing the explain/execution
plan.
We can see that there are a few scalar subqueries being used, this tends to
be an indicator of rushed SQL that can probably be rewritten to allow the
CBO full power to execute the statement effectively.
The more rows you have that drive these scalar subqueries, the more times
they get run, the more work that needs to be done.
It took just over 11,000 seconds to insert 25,000 rows
Yeah, that is slow! It's certainly 10,000 times slower than just inserting
25,000 rows.
There were 1,509,2 87,736 buffer gets associated with the instance.
That as a statement is not helpful, we care about this statement not the
instance as a whole (with no information of how long this was over)
There is something definitely wrong. This is a 2-node RAC system.
The likely truth is that the SQL is bad and it has a hugely inefficient
execution plan as a result.
FYI the insert is called more than once. It runs pretty well the first time
then gets dramatically worse.
That's no surprise, those scalar subqueries are reading from that table,
for every row in that table that get's found from one filter you seem to
read a lot of another index.
The above is the second worse case from about a month ago. There doesn't
seem to be anything wrong with SQL
I would strongly disagree. Yes, you can probably make the statement more
efficient with more appropriate indexes, but rewriting your scalar
subqueries to better handle the volume would do wonders.
For the latest run, I decided to do periodic probes of v$session wait.
It sounds like v$session_event is what you're after, it stores aggregated
information from all waits that the session experienced in its lifetime.
Each time I did I saw the library cache pin wait all with "waited short
time". V$session did not show any blocking session. Neither did the
query tailored to find blockers for this wait. I did a system state dump
...
is not in a wait:
...
Actually we can see that these waits were experienced a long time ago,
since then the session has just been on CPU (which is probably due to the
loopy reads of a lot of index which is highly cached).
...Here are the RAC stats from the run done about a month ago. This
covers a 10 hour period
...
When you are looking at things at the instance level, it is very difficult
to see the wood from the trees. Scoping is everything when it comes to
performance investigations - just like you wouldn't do an AWR report over
24 hours when you care about a process that only runs at midday for 5
minutes, you shouldn't use a full instance AWR report for something running
in one session.
Instead, you can manually query v$active_session_history with filters to
properly drill down. You can even join from v$active_session_history to
v$sql_plan to see exactly what step of the plan was being executed most of
the time.
...
The db link wait is not associated with this process.
That is the problem with using an unscoped report, there's plenty of
irrelevant details that you'd need to ignore. Although, the top event is
CPU by a long way.
In summary:
Rewrite the query to not use so many scalar subqueries
Work out why those index range scans are so highly selective
Hope this helps,
Andy
On Sun, 2 Dec 2018 at 18:17, Tim Gorman <tim.evdbt@xxxxxxxxx> wrote:
Ian,
I wrote a PL/SQL package called ASH_XPLAN
<http://evdbt.com/download/presentation-real-time-sql-tuning-delivered-at-hotsos-symposium-2015-and-utoug-training-days-2015/>
that combines DBMS_XPLAN with information from V$ACTIVE_SESSION_HISTORY to
accumulate the ASH time spent in each step of the execution plan of a
presently-running SQL statement. By running ASH_XPLAN in a separate
SQL*Plus session repeatedly, I can see which way the wind blows in a very
short time, without having to wait for the statement to complete. Of
course, please don't consider accumulated ASH time to be exactly equal to
elapsed time, as explained by Graham, Uri, and JB
<http://www.rmoug.org/wp-content/uploads/QEW_Presentations/2012/ASH%20Architecture%20and%20Advanced%20Usage.pdf>
.
I'm aware that the DBMS_SQLTUNE and SQL Monitor functionality (thanks Tim
Hall!)
<https://oracle-base.com/articles/11g/real-time-sql-monitoring-11gr1> in
views like V$SQL_PLAN_STATISTICS does this as well, but I found it fun and
educational to solve the problem myself. I was lonely for a nice PL/SQL
project that week. :)
Either way, using ASH_XPLAN or DBMS_SQLTUNE, I strongly recommend
displaying the actual elapsed time for each step of the plan in an actual
execution to determine where the most time is being spent.
Nothing really matters in optimization except elapsed time, everything
else is explanatory information.
Elapsed time (a.k.a. "life") is too short for guessing.
Hope this helps...
-Tim
On 12/1/18 22:17, MacGregor, Ian A. (Redacted sender ian for DMARC) wrote:
The table is very small about 64K. Here is an explain plan from a
month ago.
-------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows |
Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------------
| 0 | INSERT STATEMENT | | |
| 2999 (100)| |
| 1 | LOAD TABLE CONVENTIONAL | | |
| | |
| 2 | HASH UNIQUE | | 1 |
477 | 2999 (1)| 00:00:36 |
| 3 | NESTED LOOPS | | 1 |
477 | 2994 (1)| 00:00:36 |
| 4 | NESTED LOOPS | | 192 |
477 | 2994 (1)| 00:00:36 |
|* 5 | HASH JOIN | | 2 |
254 | 6 (0)| 00:00:01 |
| 6 | NESTED LOOPS | | 8 |
608 | 3 (0)| 00:00:01 |
|* 7 | TABLE ACCESS FULL | PS_PSA_RULES_HDR | 8 |
504 | 3 (0)| 00:00:01 |
|* 8 | INDEX UNIQUE SCAN | PS_PSA_RULES_LVL | 1 |
13 | 0 (0)| |
|* 9 | TABLE ACCESS FULL | PS_PSA_RULES_LN | 32 |
1632 | 3 (0)| 00:00:01 |
|* 10 | INDEX RANGE SCAN | PS_PSA_ACCTG_TA34 | 96 |
| 1473 (1)| 00:00:18 |
|* 11 | INDEX RANGE SCAN | PS_PSA_ACCTDSTGL4 | 1 |
20 | 2 (0)| 00:00:01 |
|* 12 | INDEX RANGE SCAN | PS_PSA_ACCTDSTGL4 | 1 |
14 | 2 (0)| 00:00:01 |
| 13 | SORT AGGREGATE | | 1 |
19 | | |
| 14 | TABLE ACCESS BY INDEX ROWID| PS_PSA_ACCTDSTGL4 | 1 |
19 | 3 (0)| 00:00:01 |
|* 15 | INDEX RANGE SCAN | PS_PSA_ACCTDSTGL4 | 1 |
| 2 (0)| 00:00:01 |
|* 16 | TABLE ACCESS BY INDEX ROWID | PS_PSA_ACCTG_TA34 | 1 |
350 | 1494 (1)| 00:00:18 |
-------------------------------------------------------------------------------------------------------
It took just over 11,000 seconds to insert 25,000 rows There were 1,509,2
87,736 buffer gets associated with the instance. There is something
definitely wrong. This is a 2-node RAC system. FYI the
insert is called more than once. It runs pretty well the first time then
gets dramatically worse. The above is the second worse case from about a
month ago. There doesn't seem to be anything wrong with SQL
For the latest run, I decided to do periodic probes of v$session wait.
Each time I did I saw the library cache pin wait all with "waited short
time". V$session did not show any blocking session. Neither did
the query tailored to find blockers for this wait. I did a system state
dump
-------------------------------------------------------------------------------
Oracle session identified by:
{
instance: 1 (fsprd.fsprd1)
os id: 28554
process id: 176, oracle@erp-fprd-oracle01
session id: 406
session serial #: 251
}
is not in a wait:
{
last wait: 120 min 20 sec ago
blocking: 0 sessions
current sql: INSERT INTO PS_PSA_ACCTDSTGL4 (PROCESS_INSTANCE,
BUSINESS_UNIT_PC, PROJECT_ID, ACTIVITY_ID, RESOURCE_ID, LINE_SEQ_NBR,
DEBIT_CREDIT, DST_USE, INTER_ORG_LEVEL, CONVERSION_RATE, ORG_TO_BOOK,
COMBO_STATUS, PROJECT_FLAG, IU_ANCHOR_FLG, CONTRACT_NUM, CONTRACT_LINE_NUM,
CONTRACT_PPD_SEQ, ACCT_PLAN_ID, EVENT_NUM, ACCOUNT, AL
short stack:
ksedsts()+465<-ksdxfstk()+32<-ksdxcb()+1927<-sspuser()+112<-__sighandler()<-qerixtFetch()+547<-subex1()+259<-subsr3()+183<-evaopn3()+2533<-expepr()+576<-evaiand()+51<-expeal()+23<-qerixtFetch()+800<-qerjotRowProc()+359<-qerhjInnerProbeHashTable()+491<-kdstf11001010000km()+617<-kdsttgr()+103608<-qertbFetch()+2455<-rwsfcd()+103<-qerhjFetch()+621<-qerjotFetch()+2025<-qerjotFetch()+2025<-qerghFetch()+315<-rwsfcd()+103<-qerltcFetch()+1223<-insexe()+691<-opiexe()+5632<-kpoal8()+2380<-opiodr()+917<-ttcpip()+2183<-opitsk(
wait history:
1. event: 'library cache pin'
time waited: 0.000158 sec
wait id: 7433154 p1: 'handle
address'=0x9c48940d0
p2: 'pin address'=0x5eec1bd08
p3:
'100*mode+namespace'=0x41cdd00010002
* time between wait #1 and #2: 0.000046 sec
2. event: 'library cache lock'
time waited: 0.000228 sec
wait id: 7433153 p1: 'handle
address'=0x9c48940d0
p2: 'lock
address'=0x5eeef6eb0
p3:
'100*mode+namespace'=0x41cdd00010002
* time between wait #2 and #3: 0.000516 sec
3. event: 'library cache pin'
time waited: 0.000250 sec
wait id: 7433152 p1: 'handle
address'=0x9a2fdaf10
p2: 'pin address'=0x99a42e960
p3:
'100*mode+namespace'=0x2160300010002
}
==========================================================================================
I thought the lock/pin was against an index, but the 100 indicates it is
against the table. However despite their showing up when I probed
v$session_wait they are not shown to be a significant wait. I was
surprised to see them however, because I associated such locks/pins with
DDL not DML.
The last point is that this is a 2 node RAC system, the database was
cloned to a single instance database and the program ran much more
efficiently. The longest time for an insert statement was less than an
hour.
Here are the RAC stats from the run done about a month ago. This covers
a 10 hour period
The top RAC associated wait was ranked 10th
- *• *
*Event*
*Waits*
*Total Wait Time (sec)*
*Wait Avg(ms)*
*% DB time*
*Wait Class*
DB CPU
38K
96.6
SQL*Net message from dblink
3,038
497.6
164
1.3
Network
db file sequential read
1,233,886
474.4
0
1.2
User I/O
control file sequential read
149,191
96.6
1
.2
System I/O
direct path read
71,377
89.5
1
.2
User I/O
SQL*Net more data to dblink
45,096
51.1
1
.1
Network
log switch/archive
21
43.8
2086
.1
Other
recovery area: computing obsolete files
10
22.3
2234
.1
Other
log file sync
33,439
19.2
1
.0
Commit
gc current block 2-way
42,984
16
0
.0
Cluster
The db link wait is not associated with this process.
Ian A. MacGregor
SLAC National Accelerator Laboratory
Computing Division
To offer the best IT service at the lab and be the IT provider of choice.
------------------------------
*From:* oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx>
<oracle-l-bounce@xxxxxxxxxxxxx> on behalf of Andy Sayer
<andysayer@xxxxxxxxx> <andysayer@xxxxxxxxx>
*Sent:* Friday, November 30, 2018 2:56:28 PM
*To:* dmarc-noreply@xxxxxxxxxxxxx
*Cc:* ORACLE-L
*Subject:* Re: Tuning Self-referencing Inserts
I think we need to take a few steps back.
It’s hard to explain why such a statement would cause noticeable issues if
the table is really not that large. A simple insert into <target> statement
using a select from <target table>, won’t take a silly amount of time -
it’s just the time to read the table blocks using a full tablescan then the
time to update the indexes which might be some overhead but no different to
any other insert statement of the same volume.
Perhaps this this insert is within some loop? Perhaps the self-reference
is written so that a silly execution plan is being used. It would be great
if you could share the execution plan you are seeing and the SQL being
executed. Is there PL/SQL involved?
One other thing that springs to mind that could have an impact is DML
error logging (it sounds like a situation where unique keys could give you
errors?)
As for library cache pins, this is unusual for an insert/select statement.
Things like running a PL/SQL procedure which another session is trying to
recompile while another session is executing it will bring around this
behaviour. Perhaps there is a function being called by your statement.
Again, what exactly are you seeing? This is a block so you should be able
to investigate what the blocking session is doing by checking v$session.
Hope this helps,
Andy
On Fri, 30 Nov 2018 at 18:32, MacGregor, Ian A. <
dmarc-noreply@xxxxxxxxxxxxx> wrote:
The problem is with a PeopleSoft statement which is based on a select
statement which references the table being inserted. The problem stems
from having to read and build the indexes of the table which is not that
large. The buffer gets are extremely high.
Is there generic answer to this problem. Would it help to rebuild with
a higher percent free in an attempt to have few rows per block so as to
lessen contention.
Also for library cache pins I don't understand how to reads the p3raw
value. I think the problem primarily lies with the maintenance m of the
unique index bit I am. not 100% confident.
Ian A. MacGregor
SLAC National Accelerator Laboratory
Computing Division
To offer the best IT service at the lab and be the IT provider of choice.
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