>1) I'm building a monitoring system that has to store lots of sensor
>data that I 'll have to query from time to time. I have pressure and
>temperature. Since we sample every 500 ms we will get lots of data
>after some time. Will my performance increase by making 2 tables; one
>with pressure and one with temperature? Thus when querying (eg
>pressure) mysql will only need to look in the pressure table that
>contains half the data opposed to when querying a table that contains
>pressure and temperature (and is double the size)?

Double the size? Surely you're storing something other than
temperature and pressure in the table, like date, time, which
temperature/pressure sensor, etc. Assuming you always take temperature
and pressure readings together and put them in the same table, the
other info wouldn't have to be duplicated.

If the temperature and pressure readings need DIFFERENT time stamps
(a single one for both isn't sufficiently accurate) you probably
want separate tables.

Do your queries often need pressure alone, or do they often want
temperature and pressure together?

At this rate of data generation, you should worry not only about
the QUERIES, but the data insertion as well. Using two tables means
maintaining two indexes of the data. More indexes => faster reading
data but slower inserting it.

>2) I have read that when querying the server in read mode opposed to
>read/write mode you can get a performance increase. Can this be done
>with php?

What is "querying in read mode"?

It is often true that reading a single record is faster than inserting
a single record (due to locking contention and updating indexes) but
two such queries do not substitute for each other. You cannot usually
replace reading a record with inserting one.

Also remember that any program can run infinitely fast and with zero
storage if it doesn't have to produce the correct result.

Gordon L. Burditt