Re: Why does a hard parse access data in the table in the query?

Hello,

consider the following MOS notes if on 12.1:

Recommendations for Adaptive Features in Oracle Database 12c Release 1 (Adaptive Features, Adaptive Statistics & 12c SQL Performance) (Doc ID 2187449.1)

ALERT: Oracle Recommended Adaptive Feature Configuration Parameter Settings for 12.1.0.2.0 (Doc ID 2289719.1)

Database Hang on 12c with 'Cursor: Pin S wait on X', 'Row Cache Lock' and 'Library Cache Load Lock' (Doc ID 2127483.1)

Regards
Dimitre

On 07/02/2019 11:22, Noveljic Nenad wrote:
> Maybe dynamic sampling caused by the adaptive features?
>
> Nenad
>
> https://nenadnoveljic.com/blog/
>
>
> -----Original Message-----
> From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On Behalf Of Norman Dunbar
> Sent: Donnerstag, 7. Februar 2019 11:19
> To: oracle-l_at_freelists.org
> Subject: Why does a hard parse access data in the table in the query?
>
> I get the feeling I should know this, but it seems to have aged out of the cache!
>
> I'm looking at a problem with numerous "Cursor pin S wait on X"
> problems, which I know the cause of, multiple sessions parsing (too
> often) the same statements waiting on the session doing the hard parse.
>
> That session is itself hung on "library cache lock" because there's another session doing "ALTER TABLE" elsewhere.
>
> In extracting the data to analyse this, I noticed a number of sessions doing hard parses (IN_HARD_PARSE='Y' in DBA_HIST_ACTIVE_SESSION_HIST) but they were waiting on events such as "DB file scattered/sequential read" and "Read by other session".
>
> I'm wondering why the (hard) parse needs to be reading multiple blocks (in the case of a sequential read) from a partition of one of the tables involved in the query?
>
> I could understand reading the header perhaps, but data from somewhere in the middle of the table's partitions? I checked where it was reading from and it's definitely not the header block (well, blocks, it was an 8 block read).
>
> As ever, thanks in advance for any insights.
>
>
> Cheers,
> Norm.
>
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> Norman Dunbar
> Dunbar IT Consultants Ltd
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