snippets

Getting a raw constant number of rows from oracle’s table sample function


You may of course know these two famous posts called To sample or not to sample… (part-2) about data sampling by Mark Hornick. Although very limited in scope, the two posts (imho) very well sketch why we may employ data sampling and how we may lift off table sampling in oracle.
In general, sampling is used to make a representative statement about a collection of data while only regarding a limited random selection, the sample. As long as you are ok to analyze just a sufficient subset of your 1o million rows table for an analysis, you will save your environment a lot of resources and time. On some other scenario, a limited random data selection may also serve verification or testing purposes where, however, not the representativeness but the randomness at a more or less constant sample size, determines the quality of the sample output. Again, as long as you are ok to not exceed this 15 minutes time window overnight, you will be allowed to run that live unit test on any table in question, on 1, 10 or 100 million rows.
In sql, selecting in regard to gain a representative statement will feed the sample function with a requested percentage of rows to sample from. This is what the oracle sample function already offers. Yet another sql to accept a requested actual number of rows to return, independent of the table size, is not available so far (although most people do expect exactly this behaviour when they spot the sql sample function for the first time, weird). The following text will outline a snippet of pl/sql to provide for a sample function to accept the expected number of rows as a parameter.

(more…)

Advertisements

Scheduling / descheduling linux host reboots via shutdown


Scheduling and/or descheduling linux host reboots is possible with the shutdown -r command using the time parameter (the reboot command, that I usually prefer in favour of clarity, does’nt feature the time parameter, so shutdown -r is the only choice here). Aside from discussing the quite straightforward man page of shutdown, there is two points here to register in your knowledge cells.
First, a (scheduled) shutdown -r hh24:mi execution will hangup itself into background, no need to use job-tools or an &. shutdown -r hh24:mi actually puts systemd-shutdownd in charge of serving the party, this is what you”ll want to expect to see in your running process list, looking for some command effect. Also, a running scheduled shutdown may be cancelled using shutdown -c any time before hh24:mi. Note however, that from around five minutes before hh24:mi, you’ll you’ll no longer be allowed to login the machine, essentially impeding any further control from your side.

(more…)

Using the DataImportHandler XPathEntityProcessor on a Database Resultset Column


The Solr documentation for XPathEntityProcessor introduces a spezialization subtype of EntityProcessor that is primarily depicted to process data (to be) imported from xml/http-datasources (for example, Usage with XML/HTTP Datasource). However, using XPathEntityProcessor on a FieldReaderDataSource instead on the original URLDataSource or !HttpDataSource (search for FieldReaderDataSource in Uploading Structured Data Store Data with the Data Import Handler) enables reading xml instances contained in columns delivered from database requests through SqlEntityProcessor.
Bewildered out of words and meanings…? Don’t worry, the following will give you a living example of how to craft the xml from an Oracle database easily and what to do on the Solr side to map the information datums into indexing fields. To me, this is really a nice example of how to employ xml in a true sense of a defined (well-forming, encoding) data exchange layer, hiding most if not all of the implementation details of xml processing on the database and on the search-engine. Note however, that this great time-to-market, through xml processing technically, always comes at a certain extra cost such that the xml-instances shall not become to large for this solution pattern. I will also use xml attributes for small size values instead of tags in the xml generation as one step of optimization.

(more…)

Rman duplicating an oracle 11g database in active state on the same host


I already published a post about rman> duplicate... a couple of years ago (Rman duplicating an oracle database in a new sid two host scenario), still on 10g at that time and using a backup set being transfered to a new host. With 11g, however, rman> duplicate... offers another option to not only restore from a source backup, leaving source online, but from an up and running database (with takes archivelog mode and some rman catalogued entries in the control file or catalogue nevetheless). Following below therefore, I’m going to show the do’s for an rman> duplicate ... from active database... in a same host scenario, on windows again, using orapwd and oradim as well as lsnrctl this time. The main difference, however, is employing the spfile clause of the duplicate command, such that rman will set up the destination spfile on its own. Only some file name mappings, actually like before, need to be specified. My main reference to review any new features was Duplicating a Database from the oracle 11g1 docs, other references, concerning errors that showed up underway, will be given below.

Ok, working on the same host, nothing is due to be done for software installation and stuff and we can immediately set up the new instance (note that source will be denoted tgt, for target and the destination aux for auxiliary, respectively). Firstly, we create a new password file for destination, with the same sysdba password as on source.

cd /d e:\oracle\product\11.2.0\dbhome_1\database
orapwd file=PWDAUX.ora ignorecase=y force=y

(more…)

Loop over sqlplus connection identifiers from in dos shell


Nearly at the same season last year, I wrote about ways and uses of piping sqlplus commands into a sqlplus session from a dos shell script (see : Piping newlined commands into sqlplus from a dos shell). The pattern worked fairly well so far such that I employed the technique more often lately to deploy code changes around database instances. However, when the piping stuff relieved me from writing that show errors and exit over and over again for every sql-script, I was still forced to duplicate script code over any database identifier and database session in action.

There was a point, eventually, when I felt I need to take this further, introduce a loop like in any programming language, in fact suffer this dos shell syntax quirksmode (a ss64 syntax redirection article was of great help) and just do it. Ok, it took some time to grab this enabledelayedexpansion thing, the array syntax and so on but here you go. Regard the doubled percent signs prefixing the loop variable, the array for the tns variable and again, the tremendous clear text password use within a script. Ahem, yes, compared to the antecessor article, you do not need the ampersand sign anymore, when the individual echo comands occupy own distinct lines (changed it over there).

@echo off
setlocal enabledelayedexpansion
set USR=joe
set PWD=joe
set TNS=(db1 db2)

chcp 1252
cd /D d:\database

for %%I in %TNS% do (
  (echo set timing off
    echo select '%USR%/%%I' as connection from dual;
    echo @package1.pck
    echo @package2.sql
    echo @package3.sql
    echo exit
  ) | sqlplus -s %USR%/%PWD%@%%I
)

Have fun, Peter