How can I optimize a dynamic search query in Oracle

Question

I am writing a stored procedure to perform a dynamic search that spans 10+ database tables. With millions of records in each table and a dynamic set of search parameters*, I am having some trouble optimizing the procedure.

Is there a "best practice" for building these kinds of queries? E.g. Use strings to build a dynamic query, use a huge list of IF THEN .. ELSE statements, etc? Can anyone provide a simple example or point me to some literature that will help? Here's some psuedocode for the stored procedure I am developing, which accepts a collection of parameters and a ref cursor.

v_query = "SELECT .....";
v_name = ... -- retrieve "name" parameter from collection
if v_name is not null then
   v_query := v_query || ' AND table.Name = ' || v_name;
end if;
open search_cursor for v_query;
...

*By "dynamic set of search parameters," I mean that I pass in a collection of parameters. I figured this would be easier than making the caller pass in 20 parameters if they only want to search on one.

Many thanks!

Answer 1

We've had a similar requirement for one of our clients. They have half a dozen tables with millions of rows, and they wanted adhoc search capability on most of the columns.

The solution was a separate package for each table, which would take the search criteria and construct the SQL to run the search. We took advantage of the old system that was being replaced, to discover what the most common types of searches the users were doing, and made sure that those searches ran the best, by tuning the queries that were being generated (supported by the strategic use of indexes). Because each package was only responsible for queries against one table, it could have specific code designed to work with that table (including the odd hint, in a few rare cases).

One question/problem that you'll need to address is, do you hard-code the criteria (e.g. WHERE SURNAME='SMITH') or use bind variables? Using bind variables reduces hard parsing, which reduces load on the database server; however it can be impractical to use bind variables when the SQL is dynamically generated. The way we ended up going was to set CURSOR_SHARING=FORCE (which has its own disadvantages) which was a reasonable compromise in our case.

Answer 2

Read http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:6711305251199

Answer 3

Except in very particular cases, I don't think it is advisable (or even possible) to try to generate an optimized query. My advice is not to use dynamic SQL if you can : hard to read, hard to debug, hard to optimize, hard to maintain.

First, write a generic query that will work with any parameter sent to your procedure. According to your example, that would give something like :

SELECT * FROM table WHERE ((v_name IS NULL) OR (table.Name=v_name));

As you see, you could easily add other parameters to this query without using dynamic SQL. This query is much easier to read and debug. Ask your DBA for optimization tips.

Then, if you have a particular set of parameters that you know are often passed together, you could write a particular query for this set that you could specifically optimize. Pseudocode :

IF particular_set
THEN
    /* Specific query */
ELSE
    /* Generic query */
END IF;

The difficult part is to try not to have too many specific queries here, or you could fall into a maintenance hell.

Answer 4

There are problems with using the static query approach; also be very careful about using the CURSOR_SHARING=FORCE option - it can really raise hell with your system if you haven't done a coverage test to ensure that all your other queries will work the way you want.

Problems with static queries:

1. The (x is null or x = col) predicates tend to kill any chance of using indexes. Since the query plan is computed at the time query is parsed the first time, the indexes you use will be based on the values for the first run of the query; later runs, which may not constrain on the same columns, will still use the same indexes.

2. Having one static statement with substitution variables will prevent the optimizer from making an intelligent choice about which index to use based on the data distribution. In a dynamic query (or in the first run of a query with bind variables), Oracle will see how selective your constraint is; a highly selective constraint will become a prime candidate for index use. For example, if your table had a row for every person in the U.S., STATE='Alaska' will be much more likely to use the index on STATE than STATE='California'.

Of course, in both these cases, if the dynamic columns in your WHERE clause are not indexed anyway, it doesn't matter, although I'd be surprised if that were the case in a database the size you're talking about.

Also, consider the real cost of all that hard parsing. Yes, hard parses serialize system resources, which makes them expensive, but only in the context of high volume queries. By their nature, ad-hoc queries do not get run very often. The cost you pay for all the hard parses you incur in an entire day will likely be hundreds of times less than the cost of a single query that uses the wrong indexes.

In the past, I've implemented these systems pretty much like you've done here - a base query portion, then iterating over a constraint list and adding WHERE clause predicates. I don't think it's hard for someone to maintain or understand, especially if you're talking about constraints that don't involve adding a lot of subqueries or extra tables to the FROM clause.

One thing to consider: If this system is primarily an offline one (in other words, not constantly being updated or inserted into - populated by periodic loads of bulk data), you may want to look into using BITMAP indexes. Bitmap indexes differ from regular b-tree indexes in that multiple indexes on a single table can be used simultaneously, and bitmap indexes are much, much smaller on disk than b-trees. They work very well for applications like this - where you will have a variety of constraints that can't be defined at design time. You will only want to put bitmap indexes on columns that have relatively few distinct values - say, one value constitutes no less than 1/1000 of the table - so don't use bitmaps on unique columns.

However, the downside is that bitmap indexes will noticeably degrade the performance of inserts and updates. The best practice for bitmaps is to use them in data warehouse applications, and they are dropped prior to loads and recreated afterwards.