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

[Martin Schoenbeck]

Hi Marc,

"Marc Wöhlken [quadracom]" schrieb:

>> So explain exactly tells whether and how an index will be used.
> It does only indicate whether or not indexes are used BUT NOT if a query
> will benefit from them!

Of course not. Otherwise you wouldn't need to define indexes, because the
RDBMS could do it by himself. ;-)

>>> 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.
>> 
>> Maybe, but then explain will tell exactly that.
> That's not quite true as explain does not take into account the stored
> data. Explain just works on the table structure and indexes.

Maybe you simply misunderstood it. Explain tells you when mysql is going to
do a full table scan instead of using an index due to such a condition.

Martin
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