[TYPO3-dev] Testing TYPO3 caching vs.[FE][pageCacheToExternalFiles]

[Ries van Twisk]

Martin Schoenbeck wrote:

>Hi Marc,
>
>"Marc Wöhlken [quadracom]" schrieb:
>
>  
>
>>Explain is always a good starting point for query optimization.
>>It naturally can't take into account how exaclty indexes will be used.
>>    
>>
>
>Why do you and other think, explain wouldn't know how indexes will be used?
>I'm not 100% sure for mysql, but normally explain simply calculates the
>plan how to execute the given statement and then simply dumps this plan
>instead of executing it. So explain exactly tells whether and how an index
>will be used.
>  
>
Fully agreed...... that is exacly what explain is for, to show the plan 
used.

>  
>
>>AFAIK indexes are for example not used for disk i/o if the result
>>contains more than 1/3 of the total datasets as a complete read of the
>>corresponding db file should be faster than doing random disk acces
>>based on indexes.
>>    
>>
>
>  
>
This highly depends on settimgs in the RDBM, PosthreSQL has some nice 
features to optimize it.

>Maybe, but then explain will tell exactly that. There are really situations
>where a full table scan is nothing bad. But because that depends on the
>data you can't do index optimization for all situations. What you typically
>can do is an optimization which works for big sites and then verify that it
>doesn't hurt small sites.
>  
>
Indeed... like most manuals will tell you need large datasets befire a 
RDBM will really start to use indexes.
I know for example that PostgreSQL will sometimes not use a index while 
a index was some key. At first this
looks weird but often it makes a lot of sence after analyzing it. Often 
the table is very small for example and this
the RDBM will not benifit of using the index first, rather it quickly 
scans the table which is often faster.

Ries

> 
>Martin
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>  
>


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Ries van Twisk
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