select
s.module
,ceil(max(s.rows_processed/s.fetches)) rows_per_fetch
from v$sql s
where
s.rows_processed>100
and s.executions >1
and s.fetches >1
and s.module is not null
and s.command_type = 3 -- SELECTs only
and s.program_id = 0 -- do not account recursive queries from stored procs
and s.parsing_schema_id!=0 -- <> SYS
group by s.module
order by rows_per_fetch desc nulls last
/
Tag Archives: troubleshooting
Revisiting buffer cache Very Large Object Threshold
If you turn on NSMTIO tracing you will see references to VLOT:
qertbFetch:[MTT < OBJECT_SIZE < VLOT]: Checking cost to read from caches (local/remote) and checking storage reduction factors (OLTP/EHCC Comp)
I had said you could ignore VLOT and Frits Hoogland pointed out that tracing showed it had some impact, so let me clarify:
VLOT is the absolute upper bound that cached reads can even be considered.
This defaults to 500% of the number of buffers in the cache i.e.
_very_large_object_threshold = 500
While this number is not used in any calculations, it is used in two places as a cutoff to consider those calculations
1) Can we consider using Automatic Big Table Caching (a.k.a. DWSCAN) for this object?
2) Should we do a cost analysis for Buffer Cache scan vs Direct Read scan on tables larger than the MTT?
The logic for tables above the calculated medium table threshold (MTT) and that are NOT part of searched DMLs and are NOT on Exadata with statistics based storage reduction factor enabled (_statistics_based_srf_enabled) is:
- If _serial_direct_read == ALWAYS, use Direct Read
- If _serial_direct_read == NEVER, use Buffer Cache
- If _serial_direct_read == AUTO and #blocks in table < VLOT, use cost model
- Else use Direct Read “qertbFetch:DirectRead:[OBJECT_SIZE>VLOT]”
In practice 5X buffer cache is so large the cost based decision will come to the same conclusion anyway – the default VLOT simply saves time spent doing the analysis.
For example, I got a quick count of the number of blocks in non-partitioned TPC_H Scale 1 lineitem
select segment_name,sum(blocks),sum(bytes) from user_extents where segment_name='LINEITEM'
and created my buffer cache to be exactly the same size. With this setup, setting _very_large_object_threshold=100 bypassed the cost model and went straight to DR scan, while setting it to 200 forced the use of the cost model.
The moral of this is that the default value of VLOT rarely changes the decisions made unless you reduce VLOT to a much smaller multiplier of the cache size and can start to see it cause a few more of your larger buffer cache scans move to direct read when they are no longer eligible for cost analysis. If you wish to stop some of the largest buffer cache scans from happening you would need to set _very_large_object_threshold less than 200.
Tracing Hybrid Columnar Compression in an offload server
I had previously commented on how to use the FPLIB facility in a trace event but the question came up today of how to trace HCC in an offload server. The facility name in this case is ADVCMP (Advanced Compression) and the hierarchy is:
ADVCMP_MAIN
ADVCMP_COMP
ADVCMP_DECOMP
No compression occurs on the cell so we are only interested in the decompression tracing in an offload server.
So in this case the correct syntax is [facility.event] so:
cellcli -e 'alter cell offloadgroupEvents = "trace[advcmp.advcmp_decomp.*] disk=lowest"'
Using trace events with an offload server
I’ve noticed several people who were familiar with using trace events with cellsrv, were uncertain about how to use tracing the new offload server architecture. So whereas in the past you could have added tracing to SmartScan processing with:
> cellcli -e 'alter cell events = "trace[fplib.sage_data] disk=lowest, memory=lowest"'
the new syntax to turn it on is:
> cellcli -e 'alter cell offloadgroupEvents = "trace[fplib.sage_data] disk=lowest, memory=lowest"'
and to turn it off:
> cellcli -e 'alter cell offloadgroupEvents = "trace[fplib.sage_data] off"'
Do not use anything higher than “disk=medium, memory=medium” since the higher levels of tracing will cause the heartbeat monitor to fail with timeout issues.
The SmartScan trace events available under the fplib (Filter Processing Library) facility are:
fplib.fplib_main, fplib.sage_cache, fplib.sage_txn, fplib.sage_data, fplib.sage_index
In the cell trace directory hierarchy each offload server will have its own trace directories
diag/asm/cell/SYS_122010_150220/trace
diag/asm/cell/SYS_112331_141117/trace
In a future post I will cover how to use named offload groups to isolate the traces for a single session.
Roger MacNicol,
Data Storage Technology Group
A couple of well-known but often forgotten things for PL/SQL developers
1. Don’t forget always add NO_DATA_FOUND exception handling, when you doing “select into” in code which can be called as from PL/SQL, as from SQL.
A little example:
Suppose we need to create a function, which would call some procedure:
create or replace procedure p_nested as a int; begin select 1 into a from dual where 1=0; end; / create or replace function f_no_data_found return varchar2 as begin p_nested; return 'ok'; end; /
When we call this function in PL/SQL, it will raise NO_DATA_FOUND and we will see it:
SQL> exec dbms_output.put_line(f_no_data_found); BEGIN dbms_output.put_line(f_no_data_found); END; * ERROR at line 1: ORA-01403: no data found ORA-06512: at "XTENDER.P_NESTED", line 4 ORA-06512: at "XTENDER.F_NO_DATA_FOUND", line 3 ORA-06512: at line 1
But it doesn’t when we call it in SQL, because it’s normal for SQL: it’s just like a result of scalar subquery that returns nothing – NULL:
SQL> set null "NUL" SQL> col ndf format a10 SQL> select f_no_data_found ndf from dual; NDF ---------- NUL 1 row selected.
So if you want the function to behave the same way in PL/SQL and SQL, just add exception handling with reraising another exception or just return null.
It must be at the level of reflexes – “select into” → “exception when no_data_found”
Otherwise, later, when code become a big and difficult, you can get unstable hidden error.
2. Exceptions raised in a declaration section or in default parameters assigning will never be handled in exception section of the same level
Let’s take a look at a very simple example:
An example of exception in default parameter assigning
[sourcecode language=”sql”]
SQL> create or replace function f_value_error return int is
2 begin
3 raise value_error;
4 return 1;
5 end;
6 /
Function created.
SQL> create or replace function f(i int:=f_value_error) return varchar2 is
2 begin
3 return ‘ok’;
4 exception when others then
5 return dbms_utility.format_error_backtrace;
6 end;
7 /
Function created.
SQL> set serverout on;
SQL> begin
2 dbms_output.put_line(‘From f: ‘||chr(10)||f);
3 dbms_output.put_line(‘****************************’);
4 exception when others then
5 dbms_output.put_line(‘****************************’);
6 dbms_output.put_line(‘From higher level:’||chr(10)||dbms_utility.format_error_backtrace);
7 dbms_output.put_line(‘****************************’);
8 end;
9 /
****************************
From higher level:
ORA-06512: at "XTENDER.F_VALUE_ERROR", line 3
ORA-06512: at line 2
****************************
PL/SQL procedure successfully completed.
[/sourcecode]
[collapse]
As you can see, there are two problems:
1. an exception was handled at higher level
2. the error backtrace does not show the call of the function “F”.
If the exception was caused in the declaration, we would see the “correct” backtrace, but exception would be still handled at higher level only:
In the declaration
[sourcecode language=”sql” highlight=”25″]
SQL> create or replace function f(i int:=null) return varchar2 is
2 l_i int:=nvl(i,f_value_error);
3 begin
4 return ‘ok’;
5 exception when others then
6 return dbms_utility.format_error_backtrace;
7 end;
8 /
Function created.
SQL> set serverout on;
SQL> begin
2 dbms_output.put_line(‘From f: ‘||chr(10)||f);
3 dbms_output.put_line(‘****************************’);
4 exception when others then
5 dbms_output.put_line(‘****************************’);
6 dbms_output.put_line(‘From higher level:’||chr(10)||dbms_utility.format_error_backtrace);
7 dbms_output.put_line(‘****************************’);
8 end;
9 /
****************************
From higher level:
ORA-06512: at "XTENDER.F_VALUE_ERROR", line 3
ORA-06512: at "XTENDER.F", line 2
ORA-06512: at line 2
****************************
PL/SQL procedure successfully completed.
[/sourcecode]
[collapse]
Sometimes it’s not so dangerous, but last week I was investigating a complex case for this reason: one function when called in SQL throws strange exception, but in PL/SQL it works fine.
The exception was:
SQL> select PKG1.F(1,0,0,1275) from dual;
select PKG1.F(1,0,0,1275) from dual
*
ERROR at line 1:
ORA-06553: PLS-801: internal error [1401]
And the function has many functions calls in default parameters initialization, so I couldn’t even find out which one contains a root problem.