Tag: syntax

How to use DBMS_STATS DIFF_TABLE_STATS functions

In 11g, Oracle introduced the DBMS_STAT.DIFF_TABLE_STATS functions to help you compare two sets of statistics for a table along with all its dependent objects (indexes, columns, partitions).

There are three versions of this function depending on where the statistics being compared are located:

  • DBMS_STAT.DIFF_TABLE_STATS_IN_HISTORY (compares statistics for a table from two timestamps in the past)
  • DBMS_STAT.DIFF_TABLE_STATS_IN_PENDING (compares pending statistics and statistics as of a timestamp or statistics from the data dictionary)
  • DBMS_STAT.DIFF_TABLE_STATS_IN_STATTAB (compares statistics from a user statistics table and the data dictionary, from two different user statistics tables, or a single user statistics table using two different STATSIDs)

The functions return a report that has three sections:

  1. Basic table statistics
    The report compares the basic table statistics (number of rows, blocks, etc.).
  2. Column statistics
    The second section of the report examines column statistics, including histograms.
  3. Index Statistics
    The final section of the report covers differences in index statistics.

Statistics will only be displayed in the report if the difference in the statistics exceeds a certain threshold (%). The threshold can be specified as an argument to the functions (PCTTHRESHOLD); the default value is 10%. The statistics corresponding to the first source, typically the current table stats in the data dictionary, will be used to compute the differential percentage.

The functions also return the MAXDIFFPCT (a number) along with the report. This is the maximum percentage difference between the statistics. These differences can come from the table, column, or index statistics.

Let’s look at an example.
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JEFF Talks From Kscope18

The first day of the ODTUG Kscope conference is always symposium Sunday. This year’s Database symposium, organized by @ThatJeffSmith, consisted of multiple, short, rapid  sessions, covering a wide variety of database and database tool topics, similar to Ted Talks but we called then JEFF Talks!

I was lucky enough to present 3 of this year’s JEFF Talks that I thought I would share on my blog since there wasn’t a way to uploaded to the conference site.

In the first session I covered  5 useful tips for getting the most out of your Indexes, including topics like reverse key indexes, partial indexes, and invisible indexes.

Next up was my session on JSON and the Oracle Database. In this session, I covered topics like what data type you should use to store JSON documents (varchar2, clob or blob) the pros and cons of using an IS JSON check constraint, and how to load, index, and query JSON documents.

In my finally JEFF talk I covered some of the useful PL/SQL packages that are automatically supplied with the Oracle Database. Since the talk was only 15 minutes I only touched on 4 of the 300 supplied packages you get with Oracle Database 18c but hopefully it will give you enough of a taste to get you interested in investigating some of the others!

 

 

New Top-N Queries and Pagination Syntax

At the RMOUG Training Days, a couple of weeks, ago @GeraldVenzl and I delivered a session demonstrating what it would take to get a REST enabled, web-based application up and running with Oracle Database 12c. During the session Gerald asked me to write a query to calculate the top 10 customers we had in terms of their total spend. Below is the query I came up with.

SELECT c.c_name, 
       c.c_custid loyaltyCardNo, 
       t.city, 
       t.state,
       SUM(salesAmount) total
FROM   customers c,
       t
WHERE  c.c_custid = t.loyaltyCardNo
GROUP BY c.c_name,
         c.c_custid,
         t.city,
         t.state
ORDER BY total DESC
FETCH FIRST 10 ROWS ONLY;

After the session I got a number of questions regarding the “FETCH FIRST 10 ROWS ONLY” syntax that I used, so I thought it would be worth explaining what it is and what happens under the covers when you execute it.

What is FETCH FIRST X ROWS ONLY?

Continue reading “New Top-N Queries and Pagination Syntax”

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