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pySyncOracleStandby – Simple sync service for Oracle manual standby

Posted on January 7, 2021 by Sayan Malakshinov Posted in oracle Leave a comment

I created this simple service a couple of years ago. It’s pretty simple, small and intuitive Python app, so you can easily modify it to suit your own needs and run on any platform: https://github.com/xtender/pySync

manual standby oracle Oracle SE oracle standard edition

Format SQL or PL/SQL directly in Oracle database

Posted on December 23, 2020 by Sayan Malakshinov Posted in java, oracle, PL/SQL, SQL, SQL*Plus, SQL*PLus tips Leave a comment

Obviously we can format/beautify SQL or PL/SQL code using external tools, but sometimes it would be more convenient to format it directly in database, for example if we want to see few different sql_text’s from v$sqlarea. And thanks to Oracle SqlCL and Oracle SQL Developer, we can easily use oracle.dbtools.app.Format function from their Java library dbtools-common.jar, so if you use SqlCL or SQL Developer, you can use the same formatting options.

1. load appropriate java library into Oracle

You may have already installed SQLDeveloper or SqlCL on your database server, just check $ORACLE_HOME/sqldeveloper or $ORACLE_HOME/sqcl directories. If – not, you need to download appropriate SqlCL version that matches your java version in Oracle. For 12.2 – 19.8 you can download latest SqlCL 20.3. In fact we need just dbtools-common.jar from lib directory. I put it into $ORACLE_HOME/sqlcl/lib directory on the server and load it using loadjava:

Continue reading→
beautifier pl/sql formatter sql format SQL*Plus sqlformatter

Funny friday Oracle SQL quiz: query running N seconds

Posted on December 11, 2020 by Sayan Malakshinov Posted in curious, Funny, oracle, quiz, SQL Leave a comment

Write a pure SQL query with PL/SQL that stop after :N seconds, where :N is a bind variable.

My solution

with v(start_hsecs, delta, flag) as (
  select
    hsecs as start_hsecs, 0 as delta, 1 as flag
  from v$timer
  union all
  select
    v.start_hsecs,
    (t.hsecs-v.start_hsecs)/100 as delta,
    case when (t.hsecs-v.start_hsecs)/100 > :N /* seconds */ then v.flag*-1 else v.flag+1 end as flag
  from v, v$timer t
  where v.flag>0 and t.hsecs>=v.start_hsecs
)
select delta
from v
where 0>flag
/
--end

[collapse]
SQL> var N number
SQL> exec :N := 3 /* seconds */;

PL/SQL procedure successfully completed.

SQL> select...

     DELTA
----------
      3.01

1 row selected.

Elapsed: 00:00:03.01

Another my solution using sys.standard.current_timestamp, so some internal pl/sql…:

select count(*) from dual 
connect by sys.standard.current_timestamp - current_timestamp <= interval'3'second;
funny quiz sql

Serial Scans failing to offload

Posted on October 13, 2020 by Roger MacNicol Posted in adaptive serial direct path reads, cell_offload, oracle, SmartScan 1 Comment

Very Large Buffer Cache

We’ve observed databases with very large buffer caches where Serial Scans don’t make use of Smart Scan when that would have executed faster: improvements to the decision making for Serial Scans have been made under bug  31626438. This fix is back-portable.

A key difference between PQ and Serial is that as part of granule generation PQ sums the sizes of all the partitions that have not been pruned and passes that total size to the buffer cache decision making logic. Because the entire size to be scanned is considered, we make an accurate determination of smart scan benefits and the risk of cache thrashing.

Serial Scans on partitioned tables do not involved the coordinator and have no opportunity to get the larger picture, instead they start work immediately so each partition is considered one at a time and only that one partition’s size is considered by the decision for using Buffer Cache or Direct Read (and hence offload). In the presence of very large buffer caches any given partition can fail the “Is Medium” test (or even the “Is Small” test) and so not get offloaded.

In order to avoid this situation an upper bound of 100MB for using a buffer cache scan has been implemented for any serially scanned segment that:

  • isn’t using Automatic Big Table Caching (ABTC).
  • hasn’t had the Small Table parameter changed to a non-default value.

Any partitions larger than 100 MB will now automatically use Direct Read and hence offload on Exadata.

See also: Part 1

See also: Part 2

NSMTIO: kxfxghwm:[HWM_NOT_FOUND]

Another case to watch out for is when NSMTIO tracing shows HWM_NOT_FOUND and then choosing a Buffer Cache scan when a Direct Read offloaded scan would have been faster. This can happen when a PQ query gets executed serially (NB: this is NOT the downgrade to serial case, this is still PQ but on a single thread). In this case the coordinator again does not have the opportunity to process all the partitions and as part of that gather the High Water Mark (HWM) for each segment and checkpoint them so we fall back on buffer cache scans. A fix for this is currently being investigated.

Mixed Block Sizes

I have consistently advised against mixing block sizes in a database without a compelling reason backed up by empirical evidence, but for those who must the “Is Medium Table” logic for whether to use buffer cache or direct read has been improved when the database has more than one block size in use. This is tracked by bug 24655250 and fixed in 20.1.

See also  Random thoughts on block sizes

direct path reads oracle Roger MacNicol SmartScan

Simple function returning Parallel slave info

Posted on September 15, 2020 by Sayan Malakshinov Posted in oracle, parallel, query optimizing, SQL, statistics, troubleshooting Leave a comment

You can add also any information from v$rtsm_sql_plan_monitor if needed

create or replace function px_session_info return varchar2 parallel_enable as
   vSID int;
   res varchar2(30);
begin
   vSID:=userenv('sid');
   select 
           to_char(s.server_group,'fm000')
    ||'-'||to_char(s.server_set,'fm0000')
    ||'-'||to_char(s.server#,'fm0000')
    ||'('||s.sid||','||s.degree||'/'||s.req_degree||')'
    into res
   from v$px_session s 
   where s.sid=vSID;
   return res;
exception when no_data_found then
   return 'no_parallel';
end;
/

Simple example:

select--+ parallel
  px_session_info, count(*)
from sys.obj$
group by px_session_info
/
PX_SESSION_INFO           COUNT(*)
------------------------  --------
001-0002-0001(630,2/2)     38298
001-0002-0002(743,2/2)     34706
oracle parallel presentations troubleshooting

Android Oracle Client 2.0

Posted on June 28, 2020 by Sayan Malakshinov Posted in Android, java, oracle, SQL Leave a comment

I’ve just released new version of my Simple Android Oracle Client.

New features:

  • Supported Oracle versions: 11.2, 12.1, 12.2, 18, 19, 20.
  • SQL Templates: now you can save and load own script templates
  • Server output (dbms_output)
  • Export results as JSON, CSV and HTML files (long tap on results)
  • Copy results to the Clipboard as JSON or CSV

I use it just for basic troubleshooting and small fixes, but, please, let me know if you need anything else.
Screenshots:

android android oracle client java oracle client

Smart Scan and Recursive queries

Posted on March 5, 2020 by Roger MacNicol Posted in adaptive serial direct path reads, CBO, hints, oracle, parallel, SmartScan, trace, troubleshooting Leave a comment

Since Christmas I have been asked to investigate two different “failures to use Smart Scan”. It turns out they both fell into the same little known restriction on the use of Direct Read. Smart Scan critically depends on Direct Read in order to read the synthetic output blocks into private buffers in PGA so with Direct Read disabled Smart Scan is also disabled. In these two cases the restriction is on using Direct Read on Serial Recursive queries.

Case 1: Materialized View Refresh

A customer asked me to investigate why his MView refresh was running slowly and was failing to use Smart Scan. He had used 'trace[NSMTIO] disk=highest' which showed the cause as:

Direct Read for serial qry: disabled(::recursive_call::kctfsage:::)
Continue reading→
direct path reads hints oracle Roger MacNicol SmartScan troubleshooting

PL/SQL functions and statement level consistency

Posted on December 30, 2019 by Sayan Malakshinov Posted in deterministic functions, oracle, PL/SQL, PL/SQL optimization, query optimizing, SQL Leave a comment

You may know that whenever you call PL/SQL functions from within SQL query, each query in the function is consistent to the SCN of its start and not to the SCN of parent query.

Simple example:

create table test as 
  select level a, level b from dual connect by level<=10;

create or replace function f1(a int) return int as
  res int;
begin
  select b into res 
  from test t 
  where t.a=f1.a;
  dbms_lock.sleep(1);
  return res;
end;
/

As you can see we created a simple PL/SQL function that returns the result of the query select b from test where a=:input_var

But lets check what does it return if another session changes data in the table:

-- session 2:
begin
    for i in 1..30 loop
      update test set b=b+1;
      commit;
      dbms_lock.sleep(1);
    end loop;
end;
/
-- session 1:
SQL> select t.*, f1(a) func from test t;

         A          B       FUNC
---------- ---------- ----------
         1          1          1
         2          2          3
         3          3          5
         4          4          7
         5          5          9
         6          6         11
         7          7         13
         8          8         15
         9          9         17
        10         10         19

10 rows selected.

As you can see we got inconsistent results in the column FUNC, but we can easily fix it using OPERATORs:

CREATE OPERATOR f1_op
   BINDING (INT) 
   RETURN INT 
   USING F1;

Lets revert changes back and check our query with new operator now:

--session 1:
SQL> update test set b=a;

10 rows updated.

SQL> commit;

Commit complete.

-- session 2:
begin
    for i in 1..30 loop
      update test set b=b+1;
      commit;
      dbms_lock.sleep(1);
    end loop;
end;
/

-- session 1:
SQL> select t.*, f1(a) func, f1_op(a) op from test t;

         A          B       FUNC         OP
---------- ---------- ---------- ----------
         1          2          2          2
         2          3          5          3
         3          4          8          4
         4          5         11          5
         5          6         14          6
         6          7         17          7
         7          8         20          8
         8          9         23          9
         9         10         26         10
        10         11         29         11

10 rows selected.

As you can see, all values in the column OP are equal to the values of B, while, in turn, function F1 returns inconsistent values.

operators oracle pl/sql functions pl/sql optimization

Just short note for myself: OJPPD limitations

Posted on December 2, 2019 by Sayan Malakshinov Posted in CBO, collect, oracle, SQL, troubleshooting Leave a comment

As of Oracle 19c OJPPD doesn’t support connect-by and TABLE():

OJPPD: OJPPD bypassed: query block contains START WITH/CONNECT BY.
OJPPD: OJPPD bypassed: View contains TABLE expression.
cbo JPPD ojppd

My presentations from RuOUG meetups

Posted on November 20, 2019 by Sayan Malakshinov Posted in oracle Leave a comment

I forgot to share my files from presentations, so I’m going to keep them here:

8-sql-2596013-Использование-некоторых-новых-возможностей-SQLCBO-в-12сDownload
Intra-block-row-chaining(RuOUG)Download
Intra-block-row-chaining(RuOUG).pptxDownload
Когда-выбирается-план-с-большим-COSTDownload
Sayan Malakshinov. CBO – Query Transformations(RuOUG)Download

oracle presentations
photo Sayan Malakshinov

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DEVVYOracle Database Developer Choice Award winner

Oracle performance tuning expert

UK / Cambridge

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Recent Posts

  • CBO and Partial indexing
  • Slow index access “COL=:N” where :N is NULL
  • Where does the commit or rollback happen in PL/SQL code?
  • :1 and SP2-0553: Illegal variable name “1”.
  • ORA exceptions that can’t be caught by exception handler

Recent Comments

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  • Exadata Catalogue | Oracle Scratchpad on When bloggers get it wrong – part 1
  • Exadata Catalogue | Oracle Scratchpad on Serial Scans failing to offload
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