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select for update skip locked => empty clob

Ronald DehuysserFeb 11 2025

Hi,

I'm Ronald - the developer behind JobRunr, a distributed job scheduling tool for Java.

One of the databases we support is Oracle and we noticed that using select … for update skip locked results in strange behaviour in Oracle 12c and above. When under load, Oracle DB gives a resultset where resultset.next() is true but when trying to get the value from a CLOB, an empty string is returned where there is definitely a value.

Removing the for update skip locked fixes the problem.

Here is a Github repo that reproduces the issue: https://github.com/rdehuyss/oracle-empty-string

I don't think we do something wrong, we only see this behaviour with Oracle, not with any other database. I'm looking forward to the input of the awesome Oracle community!

Comments

MartinBach-Oracle Jan 8 2025

Hi Salomon,

please have a look at this blog post written by @ulrike-schwinn-oracle :

https://blogs.oracle.com/coretec/post/easy-sql-statement-tracking-in23c

I hope this answers your question, if not, please shout!

- Martin

Solomon Yakobson Jan 8 2025

@martinbach-oracle - No, it doesn't answer my question. Article you pointed to shows uses:

SQL> alter system set sql_history_enabled=true scope=both;

And in my post I said “Works fine when enabled on system level”. My question was about

SQL> ALTER SESSION SET SQL_HISTORY_ENABLED = TRUE;

where I showed SQL history was NOT captured even though it should be based on SQL_HISTORY_ENABLED:

Modifiable **ALTER SESSION**, ALTER SYSTEM

SY.

MartinBach-Oracle Jan 8 2025

As per the article I shared the situation is as follows at the moment

  • You must enable SQL history PDB-wide (only a DBA can do that) so there's a certain level of control over the feature
  • Your session has access to the SQL history
  • If you don't want to record anything, set sql_history_enabled to false.

I'm currently assessing if that's intended behaviour (in which case the documentation should be amended) or a feature not working as it should (in which case it needs fixing). The parameter is indeed session-modifyable, but not in the sense you expected.

We'll keep you posted.

- Martin

Solomon Yakobson Jan 8 2025

Do you mean it must be enabled on system level and not on session level and all session can do is disabe it for the session?

SY.

MartinBach-Oracle Jan 14 2025

Yes,

that's correct as of Oracle Database Free 23.6.

- Martin

Solomon Yakobson Jan 14 2025

Thanks Martin, I hope this will be added to 23AI docs soon.

SY.

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Added on Feb 11 2025
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