Question

When we fire a SQL query like

SELECT * FROM SOME_TABLE_NAME under ORACLE

What exactly happens internally? Is there any parser at work? Is it in C/C++ ?

Can any body please explain ?

Thanks in advance to all.

Answer 1

Short answer is yes, of course there is a parser module inside Oracle that interprets the statement text. My understanding is that the bulk of Oracle’s source code is in C.

For general reference:

Any SQL statement potentially goes through three steps when Oracle is asked to execute it. Often, control is returned to the client between each of these steps, although the details can depend on the specific client being used and the manner in which calls are made.

(1) Parse — I believe the first action is actually to check whether Oracle has a cached copy of the exact statement text. If so, it can save the work of parsing your statement again. If not, it must of course parse the text, then determine an execution plan that Oracle thinks is optimal for the statement. So conceptually at least there are two entities at work in this phase — the parser and the optimizer.

(2) Execute — For a SELECT statement this step would generally run just enough of the execution plan to be ready to return some rows to the client. Depending on the details of the plan, that might mean running the whole thing, or might mean doing just a very small fraction of the work. For any other kind of statement, the execute phase is when all of the work is actually done.

(3) Fetch — This is when rows are actually returned to the client. Generally the client has a predetermined fetch array size which sets the maximum number of rows that will be returned by a single fetch call. So there may be many fetches made for a single statement. Of course if the statement is one that cannot return rows, then there is no fetch step necessary.