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Jun 03 2021
Clemens Bleile

Parameterized Views with SYS_CONTEXT in Oracle. How can the optimizer peek values?

By | Oracle | No Comments

There was a dicsussion a couple of years ago about a mssing feature in Oracle that the Optimizer is not peeking values when SYS_CONTEXT is being used in a predicate. I.e. specifically when using SYS_CONTEXT in Views to simulate passing parameters to views. See the following links concerning this topic: https://connor-mcdonald.com/2016/10/20/taking-a-peek-at-sys_context https://blog.jooq.org/2016/10/20/be-careful-when-emulating-parameterised-views-with-sys_context-in-oracle https://community.oracle.com/ideas/15826 E.g. take the example of Connor McDonald here: [email protected]@PDB1> create table t ( x varchar2(10), y char(100)); Table created. [email protected]@PDB1> [email protected]@PDB1> insert…

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Feb 21 2019
Oracle Team

Create a primary database using the backup of a standby database on 12cR2

By | Database Administration & Monitoring, Database management, Oracle | One Comment

The scope of this blog will be to show how to create a primary role database based on a backup of a standby database on 12cR2. Step1: We are assuming that an auxiliary instance has been created and started in nomount mode. rman target / restore primary controlfile from ‘backup_location_directory/control_.bkp’; exit; By specifying “restore primary” , will modify the flag into the controlfile, and will mount a primary role instance instead of a standby one….

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Apr 04 2017
Daniel Westermann

Can I do it with PostgreSQL? – 14 – optimizer hints

By | Database Administration & Monitoring | 4 Comments

This is a question that comes up quite often: How can I use optimizer hints in PostgreSQL as I can do it in Oracle? Well, you cant, and the reasons are this: Poor application code maintainability: hints in queries require massive refactoring. Interference with upgrades: today’s helpful hints become anti-performance after an upgrade. Encouraging bad DBA habits slap a hint on instead of figuring out the real issue. Does not scale with data size: the…

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Apr 02 2017
Oracle Team

When automatic reoptimization plan is less efficient

By | Database Administration & Monitoring | 3 Comments

By Franck Pachot . 11gR2 started to have the optimizer react at execution time when a misestimate is encountered. Then the next executions are re-optimized with more accurate estimation, derived from the execution statistics. This was called cardinality feedback. Unfortunately, in rare cases we had a fast execution plan with bad estimations, and better estimations lead to worse execution plan. This is rare, but even when 9999 queries are faster, the one that takes too…

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Dec 07 2016
Oracle Team

Oracle 12cR2: Statistics Advisor

By | Database Administration & Monitoring | No Comments

By Franck Pachot . Today at #ukoug_tech16 12:30 in hall 11A I’ll talk about Statistics Gathering Best Practice & 12cR2 Statistics Advisor Rather than taking the Optimizer Statistics Advisor rules one by one I’ll show the things to take care (some people may call it best practices) when gathering statistics and they mention the Statistics Advisor Rule. If you need a reference about all rules, you can get it from V$STATS_ADVISOR_RULES

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Sep 21 2015
Microsoft Team

SQL Saturday Paris le 05 septembre 2015 – Slides

By | Technology Survey | No Comments

La 3ème édition du SQL Saturday à Paris est terminée et je ne peux qu’être enthousiaste à la vue du nombre de personnes que cet événement attire à chaque fois et ceci un samedi (à peu près 150 personnes). Comme à chaque fois, cela a été une bonne expérience pour ma part et je la renouvellerai à coup sûr l’année prochaine dans la mesure du possible. Vous trouverez les slides des sessions que j’ai (co)animées avec Christophe Laporte et Sarah…

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Nov 11 2014
Oracle Team

AWR does not store explain plan predicates

By | Database management, Oracle | One Comment

By Franck Pachot . What is the most important part in an execution plan? It’s probably the predicate section. It helps to see implicit conversions. It helps to understand why an index is used or not. Or to see additional predicates coming from constraints. When you get an execution plan from shared pool, the dbms_xplan.display_cursor() shows the predicates. But when you retrieve a plan from the past, dbms_xplan.display_awr() does not show them. Statspack With Statspack,…

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Nov 08 2014
Oracle Team

Oracle SQL Profile: why multiple OPT_ESTIMATE?

By | Database management, Oracle | 5 Comments

By Franck Pachot . In a previous blog I’v shared my script to retrieve the OPT_ESTIMATE hints from a SQL Profile. In the example I made, I had two lines for each table: — PROFILE HINTS from dbiInSite (1) statement 4fz1vtn0w8aak: /*+ OPT_ESTIMATE(@”SEL$2CBA5DDD”, TABLE, “EMPLOYEES”@”SEL$1″, SCALE_ROWS=2) OPT_ESTIMATE(@”SEL$58A6D7F6”, TABLE, “EMPLOYEES”@”SEL$1″, SCALE_ROWS=2) OPT_ESTIMATE(@”SEL$6AE97DF7”, TABLE, “DEPARTMENTS”@”SEL$1″, SCALE_ROWS=5.185185185) OPT_ESTIMATE(@”SEL$58A6D7F6”, TABLE, “DEPARTMENTS”@”SEL$1”, SCALE_ROWS=5.185185185) */ The reason is that when the optimizer do some transformations to the query, then the query…

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