PLSQL Advanced

Author: DuyTran
Minh Doan
10,April 2012
4/3/20141
Advanced PL/SQL

Objective
4/3/20142
You should have basic knowledge about PL/SQL
and database management before reading this.
This slide will guide you through some advanced
concepts of PL/SQL like hints, execution plan, bulk
processing, …You should use this slide as reference but
remember to do testing everything before apply to your
code.

Agenda
4/3/20143
A. Flow Control
B. Bulk Processing
C. Oracle Hints
D. Resources
E. Q/A

A.1. Code optimization – LOOP & IF
4/3/2014A. Flow Control4
 Given LOOP and IF statement
for count1 in 1..2000
loop
loop
mod1 := mod(count1, 10);
sqrt1 := sqrt(count1);
if (mod1 = 0) then
if (mod2 = 0) then
sum1 := sum1 + sqrt1 + sqrt2;
end if;
end if;
end loop;
end loop;
Executed in 8.297 seconds
Bad

A.1. Code optimization – LOOP & IF(cont)
 Enhanced version
loop
loop
if (mod1 = 0) then
if (mod2 = 0) then
end if;
end if;
end loop;
end loop;
Not bad, but …

 More fine grained
loop
if (mod1 = 0) then
loop
if (mod2 = 0) then
end if;
end loop;
end if;
end loop;
Good
20 times faster than the original one !!!

 Conclusion
 Minimize the number of iterations: use EXIT to cease looping where
necessary.
 Remove any statements within a loop that could be processed outside
the loop, especially when you have nested loop.
 Specify the most probable condition first in a compound IF statement:
try to reduce the number of evaluations.

A.2. Code optimization - Recursion
 Fibonacci calculating
function rec_proc(n_limit integer)
return integer
is
begin
if (n_limit > 1) then
return n_limit + rec_proc(n_limit-1);
else return n_limit;
end if;
end;
function non_rec_proc(n_limit integer)
return integer
is
n_sum integer := 0;
begin
for i in 0.. n_limit
loop
n_sum := n_sum + n_limit;
end loop;
return n_sum;
end;
Executed in 2 seconds

A.2. Code optimization - Recursion(cont)
 Conclusion
 Elegant from coding perspective, but consume memory and are
usually slower than iterative alternative.
 Almost all recursive algorithms have a non-recursive equivalent.

B.1. Bulk processing in PL/SQL
4/3/2014B. Bulk Processing10
 Row by row processing in PL/SQL
SQL
statement
executor
SQL Engine
Procedural
statement
executor
PL/SQL Runtime Engine
PL/SQL Block
FOR rec IN emp_cur
LOOP
UPDATE emp
SET salary = …
WHERE emp_id =
rec.emp_id;
END LOOP;
Oracle Server

B.1. Bulk processing in PL/SQL(cont)
 Bulk processing with FORALL
SQL
statement
executor
SQL Engine
Procedural
statement
executor
PL/SQL Runtime Engine
PL/SQL Block
FOR rec IN emp_cur
LOOP
UPDATE emp
SET salary = …
WHERE emp_id =
rec.emp_id;
END LOOP;
Oracle Server

 Bulk processing with FORALL (cont.)
 Fewer context switches.
 Same SQL behavior.
 Use with inserts, updates, deletes and merges.
 Move data from collections to tables.

 Warning
 BEFORE &AFTER statement-level triggers fired only once per
FORALL INSERT statement.
 Make code run faster but user session will consume more PGA
memory.
→ what’s PGA memory ???

B.2. Oracle DB memory essential
 Oracle memory structure
Program GlobalArea

B.2. Oracle DB memory essential(cont)
 Oracle memory structure – more details

B.2. Oracle DB memory essential(cont)
 Memory solution
 Bulk collection with LIMIT.
CAUTION !!!
 Do not checking %NOTFOUND right after fetch.
 Do one of the following instead:
 Check %NOTFOUND at the end of the loop.
 Exit when collection.COUNT = 0 right after fetch.
 Exit when collection.COUNT < limit at the end of the loop.

B.3. Exception handling
 Got exception in FORALL statement ?
 Add SAVE EXCEPTIONS to suppress errors at statement level.
 Exception will be handled in Exception block.

B.3. Exception handling(cont)
 Typical errors with bulk processing
 Rollback segment too small …
 Cause: too many uncommitted changes, the rollback segment can not
handle all.
 Solution: may still need to use incremental commit.
 Snapshot is too old ...
 Cause: a cursor is held open too long, Oracle can no longer maintain the
snapshot information.
 Solution: open-close cursor.

B.3. Bulk processing conclusion
 Conclusion
 Should always try to execute multi-row SQL operations in PL/SQL.
 Watch out for impact on PGA memory.

C.1. SQL processing
4/3/2014C. Oracle Hints20
 SQL processing diagram
Read more: Understand explain plan

C.1. SQL processing(cont)
 SQL processing
 Oracle execute SQL in different ways:
 Full table scans.
 Index scans.
 Nested loops.
 Hash joins.
 Execution plan.
 We have more information on context than Oracle.

 Access paths for the Cost Based Optimizer
 Full table scans → SELECT /*+ FULL(e) */ FROM Employee e.
 Rowid scans.
 Index scans.
 Cluster scans.
 Hash scans.
 Sample table scans.

 Join methods
 Nested loop joins.
 Hash joins.
 Sort merge joins.
 Cartesian joins.
 Outer joins.

 Why hints are needed ?
 Incorrect/incomplete statistics.
 Keywords that slow performance down.
 Highly volatile data changes.
 Configurations outside Optimizer knowledge scope.

C.2. Oracle hints introduction
 What is Oracle hint?
 A code snippet that is embedded into a SQL statement.
 Suggest to Oracle how the statement should be executed.
Note: Hints should only be used as a last-resort if statistics were gathered and the query is still
following a sub-optimal execution plan.
 Hint syntax
--+RULE
/*+RULE */
Read more: http://psoug.org/reference/hints.html

C.2. Oracle hints introduction(cont)
 Some of the most useful hints
 Global hints: rule, first_rows, first_rows_n all_rows, driving_site.
 Table join hints: use_nl, use_hash, ordered.
 Table access hints: parallel, full, cardinality.
 Index hints: index, no_index, index_combine.

C.3. Example hints
 Example
 Example suggesting that a FULLTABLE SCAN method be used:
SELECT /*+ FULL(x) */ FROM tab1 xWHERE col1 = 10;
 Suggest that Oracle uses a specific index:
SELECT /*+ INDEX(x emp_idx1) */ ... FROM scott.emp x...
 Suggest that Oracle DOES NOT USE a specific index:
SELECT /*+ NO_INDEX(x emp_idx1) */ ... FROM scott.emp x...

C.4. Oracle Parallel Query
 Oracle Parallel Query
 PQO allows one to break-up a given SQL statement
 Its parts can run simultaneously on different processors in a multi-
processor machine
 Typical operations that can run parallel are: full table scans, sorts, sub-
queries, data loading ,…
 Advantage: improve performance of certain types of operations
dramatically on a multi-CPU system (usually be paired with full table
scans)

C.4. Oracle Parallel Query(cont)
 How to invoke Parallel query?
 Alter the table (or index) to indicate that Oracle should try to
parallelize operations performed against it
ALTERTABLE table_name PARALLEL (DEGREE 8);
 Put hints in SQL statements to indicate that Oracle should try to
execute them in parallel
SELECT /*+PARALLEL(table_alias, degree, nodes)*/ * FROM
table_name ...
Note:You must set all the INIT.ORA parameters necessary for Parallel Query to work

C.4. Oracle Parallel Query(cont)
Using OPQ, Oracle partitions the table into logical chunks. Oracle fires off parallel
query slaves (sometimes called factotum processes), and each slave simultaneously
reads a piece of the large table. Upon completion of all slave processes, Oracle passes
the results back to a parallel query coordinator, which will reassemble the data,
perform a sort if required, and return the results back to the end user.

Resources
4/3/201431
 Doing SQL from PL/SQL: Best andWorst Practices – Oracle.
 Tuning Oracle Stored Procedures – Guy Harrison, Quest Software.
 Turbo-charge PL/SQL Performance with Bulk Processing Features – Steven
Feuerstein,ToadWorld.com.
 The “Top 10” Dumbest PL/SQLThings I Have Seen or Done – Steven
Feuerstein, Quest Software.
 Oracle Documentation about MemoryArchitecture
 Understanding explain plan
 When NOT EXIST should NOT EXIST
D. Resources

PLSQL Advanced

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