Clients - Parse, Execute, Fetch - Starttime

Question

Hello guys,
i have a look at the some php scripts which are connecting to an oracle database and it looks like the following example.
-> $db = @ora_logon("scott@testdb29","tiger")
Ok here is a connect done to oracle
-> $curs = ora_open($db);
Ok a cursor is opened
-> $sql = "SELECT * FROM dept"; ora_parse($curs,$sql)
Ok the sql statement is parsed, with the previous opened cursor
-> ora_exec($curs);
Statement is executed
-> while (ora_fetch_into($curs, $results)) { $results[0]; }
Fetches from the select data
In which step does oracle starts to read the data (logical i/o or phyiscal i/o)?
Does it start at execute or does it start at the fetch?
When it starts at execute... what happens if i don't fetch the data? for example if oracle had to sort the data in the pga...
When it doesn't start at execute... what is done on the execute command?
Are these steps the same in every client (for example sqlplus) or can it vary?
Thanks and Regards
Stefan

Answer 1

Hello Khurram,
ok no problem ... but the php documentation does not contain this information :-(
I think that is a general question... i found a german pdf which describes the sql phases of 8i.
I am translating some parts of it:
http://members.aon.at/hermann.zauner/Oracle8i.pdf
Point 2.1 SQL
-> 2.1.2.1The cursor
This point is clear
-> 2.1.2.2 The Open-Phase
Also clear
-> 2.1.2.3 The Parse-Phase
Also clear
-> 2.1.2.4 The Execute-Phase
A SCN will be observed in the cursor. This is the guarantee, that a sql statement is always consistent, that means from the sql's point of view, that the data has not changed since the start of the command.
In case of a select statement with a sort or a group by, the data will be formatted and put in some temporary area ( => i think, the creator of the pdf means the sort area or the temp tablespace)
-> 2.1.2.5 The Fetch-Phase
This phase will only be perform in case of a select statement. In the fetch phase the data will be transfered to the client. The data will be read out of data/index blocks or out of the temporary area, which was prepared in the execution phase.
-> 2.1.2.6 The Close-Phase
also clear
Ok but the pdf description is a little bit unexact in the part of the temporary areas and the fetch data.
Please correct me if i misunderstand the behaviour of the sql execution phases in case of SELECTs
- The execute phase can contain some data reads (or writes) in cases if the data must be converted (i mean sorts, groups, sums, etc..).
- A normal "select * from EMP" will initiate oracle to read the data (out of the data files into the buffer cache, if they don't already exists in there) in the fetch phase.
Maybe someone of you got more detailed information, but this was the only one i could find about that topic...
Maybe J. Lewis knows more in detail :-)
Regards
Stefan