Parsing

Here’s a quick quiz.

According to the Oracle 12.1 Database SQL Tuning Guide the first stage of parsing a statement is the Syntax Check, which is followed by the Semantic Check, followed by the Shared Pool Check. So where you do think the statement text will be while the Syntax Check is going on ?

 

 

 

 

 

 

And the answer looks like ….

 

 

 

… the shared pool. Here’s a simple test, cut-n-paste from SQL*Plus running under 12.1.0.2 in the SYS schema (I’ve also done this in the past with older versions):

 

 

 


SQL> select user frrom dual;
select user frrom dual
                  *
ERROR at line 1:
ORA-00923: FROM keyword not found where expected

SQL> select kglhdpar, kglhdadr, kglobt03, kglnaobj from x$kglob where kglnaobj like '%frrom%' and kglnaobj not like '%kgl%';

KGLHDPAR         KGLHDADR         KGLOBT03      KGLNAOBJ
---------------- ---------------- ------------- --------------------------------------------------------------------------------
00000000D1EE3880 00000000AEF43F40 bshhvh0ypcz6x select user frrom du
00000000D1EE3880 00000000D1EE3880 bshhvh0ypcz6x select user frrom du

2 rows selected.

SQL> 

So the statement is garbage and fails (or ought to be failing) on a syntax text – but I can find it in x$kglob – the library cache objects – in the SGA with a parent and child entry.

I have to say that when I first read, many years ago, that there was a syntax check it struck me that the fastest way of doing a syntax check on a “good” system would be to start with a search for the text in the library cache. If it were there (and on a good system it probably would be within a few minutes of startup) then you could have a flag that avoided the CPU cost of doing the syntax check and move straight on to the semantic (basically object and permissions) check.