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

[Martin Schoenbeck]

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.

> 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. 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.
 
Martin