12c

Johnjayking's picture

Oracle 12c – Invisible Columns!

Oracle 12c – Invisible Columns

Beginning with Oracle 12c columns may be marked “INVISIBLE” in CREATE and ALTER TABLE statements.

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Suppose you have code that uses “SELECT *” (no, really this sometimes occurs) and you need to add a column or columns? Oops! Code using “SELECT *” now breaks as does code using INSERT statements without a column list. (I know, ALL of your system’s code uses explicit column name lists in SELECT and INSERT ; this does not apply to your shop… (please excuse the sarcasm))

Jonathan Lewis's picture

Invisible Bug

At this Wednesday’s Oracle Midlands event someone asked me if Oracle would use the statistics on invisible indexes for the index sanity check. I answered that there had been a bug in the very early days of invisible indexes when the distinct_key statistic on the index could be used even though the index itself would not be considered as a candidate in the plan (and the invisible index is still used to avoid foreign key locking – even in 12c – it’s only supposed to be invisible to the optimizer).

Jonathan Lewis's picture

Uniquely parallel

Here’s a surprising (to me) execution plan from 12.1.0.2 – parallel execution to find one row in a table using a unique scan of a unique index – produced by running the following script (data creation SQL to follow):

rshamsud's picture

Accessing HCC compressed objects using an index

Problem

I came across another strange SQL performance issue: Problem was that a SQL statement was running for about 3+ hours in an User Acceptance (UA) database, compared to 1 hour in a development database. I ruled out usual culprits such as statistics, degree of parallelism etc. Reviewing the SQL Monitor output posted below, you can see that the SQL statement has already done 6 Billion buffer gets and steps 21 through 27 were executed 3 Billion times so far.

Statistics and execution plan

Jonathan Lewis's picture

Quiz Night

Here’s an execution plan from a recent OTN database forum posting:

Richard Foote's picture

Storing Date Values As Numbers (The Numbers)

In my last couple of posts, I’ve been discussing how storing date data in a character based column is a really really bad idea. In a follow-up question, I was asked if storing dates in NUMBER format was a better option. The answer is that it’s probably an improvement from storing dates as strings but it’s […]

Richard Foote's picture

Storing Date Values As Characters Part II (A Better Future)

In the previous post, I discussed how storing date values within a character data type is a really really bad idea and illustrated how the CBO can easily get its costings totally wrong as a result. A function-based date index helped the CBO get the correct costings and protect the integrity of the date data. During […]

Richard Foote's picture

Storing Date Values As Characters (What’s Really Happening)

For something that’s generally considered an extremely bad idea, I’ve lost count of the number of times I’ve come across applications that insist on storing date values as characters within the database. We’ve all seen them … I recently got called in to assist a customer who was having issues with a POC in relation […]

Jonathan Lewis's picture

Virtual Partitions

Here’s a story of (my) failure prompted by a recent OTN posting.

The OP wants to use composite partitioning based on two different date columns – the table should be partitioned by range on the first date and subpartitioned by month on the second date. Here’s the (slightly modified) table creation script he supplied:

rshamsud's picture

library cache lock on BUILD$ object

I was testing an application performance in 12c, and one job was constantly running slower than 11g. This post is to detail the steps. I hope the steps would be useful if you encounter similar issue.

Problem

In an one hour period, over 90% of the DB time spent on waiting for library cache lock waits. Upon investigation, one statement was suffering from excessive waits for ‘library cache lock’ event. We recreated the problem and investigated it further to understand the issue.

Following is the output of wait_details_rac.sql script (that I will upload here) and there are many PX query servers are waiting for ‘library cache lock’ wait event.

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