PL/SQL vs. Java - Additional Information

date:2007-03-23 15:21:08
category:Data Structures and Algorithms

Physical Tuning.

”...databases can be tuned by creating appropriate indexes, removing others, modifying the physical database schema to suit the most common types of queries, etc. So it isn’t clear whether the PL/SQL figure could be better if the database was optimised by a good DBA.”

While a good point, it doesn’t apply. In order to compare Java and PL/SQL, I had to use the same database for both implementations. It was the same Oracle 10 XE instance.

The physical design question is really about comparing two data models, not about comparing two languages for processing a single, common data model. Yes, a change to the index might make both the PL/SQL and the Java run even faster. However, any performance improvement would apply across the board to Java as well as PL/SQL.

DB Interaction.

”...it was typically faster to do as much processing as possible in the database, rather than outside of the database. This is particularly true where the external code has to make many queries to the database in the course of doing a particular piece of work. Replacing those with a single call to the database ... was a big performance gain.”

Again, a really good point, but one that doesn’t completely apply.

The core issue of PL/SQL vs. Java doesn’t matter. PL/SQL, while resident in the RDBMS server, isn’t really part of the database. PL/SQL makes SQL requests just like Java (via JDBC) is making SQL requests. Both the PL/SQL and the Java use nearly the same SQL SELECT statements. They can’t be identical because SELECT is part of the PL/SQL language, but a simple String object to Java.

In both Java and PL/SQL, we’re doing as much in the DB as possible – executing a SELECT which does simple matching and executing a SELECT which does “near-miss” matching. The PL/SQL version is slower because the non-DB work done in the PL/SQL program is slower than Java. Further, the PL/SQL run-time environment is tightly constrained to be part and parcel of the RDBMS, limiting memory and throughput.

The Bigger Picture.

From a slightly broader point of view, people regularly claim that calling a PL/SQL procedure does “more work in the database.” In this case, we have designed a moderately complex procedure, which has to exist – as a procedure – somewhere. We can’t rewrite our process to embed the logic in a SELECT statement. We can’t do more work in the database; the procedure is either PL/SQL making SQL requests or Java making SQL requests.

What happens all too often is people take something this is already a first-class feature of SQL, and write their own procedure to implement it. For example, people will write algorithms to do joins as nested lookups, or unions as a series of queries, or group-by’s in their own programs because they didn’t understand SQL, or thought they could write something faster. In these cases, writing proper SQL, and providing a sound RDBMS physical structure, is probably going to be fastest.

Any speedup from doing more work in SQL and less work in the “application” applies to a Java application as well as a PL/SQL application. The point is to make better use of the SQL query and DML features, and the physical structures in the RDBMS.

In my example, the processing can’t easily be reduced to simple SQL. It involves decision-making that is hard to code as a simple WHERE-clause. Therefore, we have two places to put the processing: PL/SQL (slow) or Java (fast).

Aside.

The SQL-For-Smarties crowd will note that multi-part matching is – technically – a kind of union. We could try to implement this processing as a union of exact matches with near-miss-but-not-exact matches. This can get very hairy, and it becomes hard to maintain because we have to do a lot of predicate calculus to work out the effect of implicitly procedural business rules.

Code Sample.

Here’s the PL/SQL package I used.

create or replace package body "MATCHSQL" is
procedure MAIN
as
   invno NUMBER;
   temp NUMBER;
   CURSOR invoice_qry( mandate DATE, mantotal NUMBER, manshiptocust NUMBER ) IS
       SELECT invno FROM INVOICE
       WHERE INVOICE.invtotal = mantotal
       AND   INVOICE.invdate = mandate
       AND   NVL(INVOICE.shiptocust,-1) = NVL(manshiptocust,-1);
   CURSOR invoice2_qry( mandate DATE, mantotal NUMBER, manshiptocust NUMBER ) IS
      SELECT invno FROM INVOICE
      WHERE INVOICE.invtotal BETWEEN mantotal-10 AND mantotal+10
      AND   INVOICE.invdate BETWEEN mandate-10 AND mandate+10
      AND   NVL(INVOICE.shiptocust,-1) = NVL(manshiptocust,-1);
   manCount NUMBER := 0;
   match NUMBER := 0;
   nearMatch NUMBER := 0;
   nonMatch NUMBER := 0;
   dupMatch NUMBER := 0;
begin
 DBMS_OUTPUT.PUT_LINE( 'Pure SQL' );
 -- Iterate through all Manifests
 FOR man IN (SELECT manno, mantotal, mandate, shiptocust FROM MANIFEST)
 LOOP
    manCount := manCount + 1;
    -- Lookup matching Invoice
    OPEN invoice_qry( man.mandate, man.mantotal, man.shiptocust );
    FETCH invoice_qry INTO invno;
    IF invoice_qry%FOUND THEN -- Exact Match?
      FETCH invoice_qry INTO temp;
      IF invoice_qry%FOUND THEN -- Duplicate?
          dupMatch := dupMatch + 1; -- Duplicate
      ELSE
          match:= match + 1; -- Exact Match
      END IF;
    ELSE
      -- No Exact Match, try a fall-back near match
      OPEN invoice2_qry(man.mandate, man.mantotal, man.shiptocust);
      FETCH invoice2_qry INTO invno;
      IF invoice2_qry%FOUND THEN -- near match?
          FETCH invoice2_qry INTO temp;
          IF invoice2_qry%FOUND THEN -- duplicate?
            dupMatch := dupMatch + 1;
          ELSE
            nearMatch := nearMatch + 1;
          END IF;
      ELSE
         nonMatch := nonMatch + 1; -- No Match
      END IF;
      CLOSE invoice2_qry;
    END IF;
    CLOSE invoice_qry;
 END LOOP;
 -- Final Report
 DBMS_OUTPUT.PUT_LINE( 'Count ' || manCount );
 DBMS_OUTPUT.PUT_LINE( '  Match ' || match );
 DBMS_OUTPUT.PUT_LINE( '  Non-Match ' || nonMatch );
 DBMS_OUTPUT.PUT_LINE( '  Near Match ' || nearMatch );
 DBMS_OUTPUT.PUT_LINE( '  Duplicate ' || dupMatch );
end MAIN;

end MATCHSQL;

The Real Speedup.

In addition to Java being faster than PL/SQL, an additional speedup comes from making fewer DB requests in the first place. That’s why I included the Pure SQL, One Dictionary and Two Dictionary results. Eliminating SQL entirely (and using Java HashMap) cuts the time down to 0.4 of the Pure SQL performance.

Here’s my golden rule:

The Fastest RDBMS Operation is The One You Don’t Do.

If it can be done outside the database, it will be faster than if it is done in the database. Clearly, there’s a fuzzy border between a sensible performance improvement and reinventing your own RDBMS in your own application program.

When it involves concurrent transactions, your program’s private cache of data is A Bad Thing ™. However, any data which is static can – and should – be cached if you need to get maximum processing speed. Static, of course, is relative. Some batch jobs run during windows in which the database isn’t “transactional”, or during which the transaction load is light or doesn’t adversely impact the correctness of results that come from cache.

if. for example, a batch job runs after midnight on yesterday’s transactions, then an in-memory cache of transaction data isn’t invalidated by processing of today’s transactions. In “Processing Rows in Batches ”, for example, one of the factors left out of the article was the confusion over what rows were actually part of a batch. The original code did lots of thrashing around to try and capture every transaction. Why not just wait until the next batch scheduling interval? Or, why have batches in the first place? Why not process rows as they arrive?

Conclusion.

DB physical tuning helps both Java and PL/SQL. Since Java is already faster than PL/SQL, physical design is still important and still helps.

Doing more work in SQL’s DML helps both Java and PL/SQL, also. Knowing SQL, and making use of the DML features is still important and still helps.

PL/SQL is slow, and I find it painful to manage, since it is relatively inflexible. I don’t have classpath, working directories, environment variables, command-line parameters in my PL/SQL environment. I do have a kind of symbolic link as my only control mechanism for introducing flexibility.

Since PL/SQL is slower and less flexible than Java, I’m forced to to conclude that PL/SQL isn’t an effective way to implement anything.

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