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Global Hints

9 Apr

Occasionally I encounter a situation when I need to affect a part of the plan that corresponds to a view, e.g.:

select *
from
(
   select v.x, x.y
   from v
) q
where q.x = 1

Such situations are resolved using global hints. Oracle offers two ways to specify a global hint: via a query block identifier (system generated or user defined) or via view aliases. System-generated query block identifiers can be obtained via dbms_xplan.display with ALL or ALIAS option (they have the form SEL$n, where n appears to be same as the depth, e.g. in our case 1 corresponds to the main query, 2 to the inline view, 3 to the view V inside that inline view) or defined by the user via qb_name hint.

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Tuning Analytic Functions

2 Apr

In general, tuning analytic functions (and more generally, all sort operations) is rather difficult. While for most poorly performing queries it’s relatively straightforward to gain some improvements  by applying “eliminate early” principle one way or another, for slow sort operations it’s rarely applicable. Usually options are limiting to rewriting a query without analytics (e.g. using self-joins or correlated subqueries to achieve the same goal) or manually resizing the workarea to reduce/eliminate the use of disk. Recently, however, I had a case where I managed to obtain an excellent performance gain using a different technique that I would like to share in this post.

The original query was selecting about 100 columns using the LAG function on one of the columns in the WHERE clause, but in my test case I’ll both simplify and generalize the situation. Let’s create a table with a sequential id, three filtering columns x, y and z, and 20 sufficiently lengthy columns.

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CPU starvation disguised as an I/O issue (yet another AWR case study)

29 Oct

In AWR analysis, what appears to be the root cause of the issue, can easily turn out to be just a symptom. Last week, Rajat sent me an AWR report which is a perfect illustration of this (thanks Rajat), I posted the key sections from this report below (sorry for less than perfect formatting — I had to manually re-format the HTML version of the report into text).


WORKLOAD REPOSITORY report for
DB Name      DB Id           Instance       Inst num        Release          RAC           Host
DSS          37220993      dss              1               10.2.0.4.0       NO            dssdbnz

                  Snap Id      Snap Time             Sessions      Cursors/Session
Begin Snap:       18471      12-Oct-12 08:30:28      131              1.5
End Snap:         18477      12-Oct-12 14:30:24      108              1.8
Elapsed:          359.93 (mins)
DB Time:          25,730.14 (mins)

Load Profile
                              Per Second      Per Transaction
Redo size:                    325,282.85      103,923.02
Logical reads:                33,390.52       10,667.77
Block changes:                1,307.95        417.87
Physical reads:               1,927.33        615.75
Physical writes:              244.65          78.16
User calls:                   391.34          125.03
Parses:                       68.14           21.77
Hard parses:                  3.33            1.06
Sorts:                        47.86           15.29
Logons:                       3.15            1.01
Executes:                     234.32          74.86
Transactions:                 3.13
% Blocks changed per Read:       3.92       Recursive Call %:      61.11
Rollback per transaction %:      24.71      Rows per Sort:         3325.52

Top 5 Timed Events
Event                               Waits          Time(s)      Avg Wait(ms)      % Total Call Time      Wait Class
free buffer waits              10,726,838      344,377     32      22.3      Configuration
db file sequential read        6,122,262      335,366      55      21.7      User I/O
db file scattered read         3,597,607      305,576      85      19.8      User I/O
CPU time                                      161,491              10.5
read by other session          2,572,875      156,821     61       10.2      User I/O

Operating System Statistics
Statistic                                 Total
AVG_BUSY_TIME                             2,093,109
AVG_IDLE_TIME                             63,212
AVG_IOWAIT_TIME                           18,463
AVG_SYS_TIME                              87,749
AVG_USER_TIME                             2,004,722
BUSY_TIME                                 16,749,988
IDLE_TIME                                 510,692
IOWAIT_TIME                               152,594
SYS_TIME                                  707,137
USER_TIME                                 16,042,851
LOAD                                      4
OS_CPU_WAIT_TIME                          ###############
RSRC_MGR_CPU_WAIT_TIME                    0
VM_IN_BYTES                               5,503,492,096
VM_OUT_BYTES                              2,054,414,336
PHYSICAL_MEMORY_BYTES                     34,288,209,920
NUM_CPUS                                  8
NUM_CPU_SOCKETS                           8

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AWR report case study: stating the obvious

9 Jul

It’s been a while since I came across an interesting and complex case. However, yesterday on an OTN forum thread I saw a case which was interesting by its simplicity. Even though it’s almost trivial on the technical level, it’s very useful to highlight typical tuning “rookie mistakes” (I can remember quite a few cases from not so long ago, when I did similar mistakes, too).

The author posts a question about “library cache: mutex X” events that are ruining performance of his 2-node RAC cluster. The original post doesn’t contain any specifics except for CPU utilization percentage on both nodes.

Within a few hours, a few replies appear, most of them either trying to shed light on this particular wait event or sharing similar experiences. I asked the original poster to provide key sections of an AWR report (workload profile, top events, database/host CPU, top SQL), which he soon did:

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