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Oracle Subquery Factoring

Oracle Database Tips by Donald Burleson

Oracle Subquery Factoring

Question:  I hear that subquery factoring is a good replacement for global temporary tables but I don't know the syntax. 

What is subquery factoring and what is the syntax for subquery factoring?

Answer:  A more common name for subquery factoring is the SQL WITH clause, an ANSI-99 standard keyword. 

The main purpose of Oracle subquery factoring is to simplify complex SQL.

Subquery factoring simplifies complex SQL by materializing complex SQL in much the same way as global temporary tables.

Also, starting in 11g R2, we see the new recursive subquery factoring using the SQL WITH clause

First, review these notes to understand how powerful subquery factoring for pre-aggregating and simplifying complex SQL queries:

****************************

Question:

Although there is an index starting with record_type, there can be many millions of rows satisfying the record_type 'PE01'. As a result of Oracle subquery factoring, full table scans are performed on the temporary table as follows:

My question is this: Is it possible to get it to create and use an index instead of full-scanning SYS_TEMP_0FD9D6608_37E723F9 twice, as a result of Oracle subquery factorying? That itself could get huge, as there are only a limited number of codes available in record_type and PE01 is likely to be the most common, accounting for about a quarter to one-half of the rows in addl_field_details.



The same POC query without Oracle subquery factoring looks like this:

and on the same database the POC query without Oracle subquery factoring performs like this:

Is there a better way for me to optimize this to avoid the full-scans caused by the Oracle subquery farctoring?

Answer:

First, to better understand Oracle subquery factoring please review this great excerpt from the book "Advanced SQL Programming" on Oracle subquery factoring:

Subquery Inline View - Oracle Subquery Factoring

In the past I've replaced WITH queries with global temporary tables because you can create indexes on them and avoid some of the slowdown associated with Oracle subquery factoring.

See my notes on tuning SQL with global temporary tables here.

Let's take a look at an example tuning SQL with global temporary tables to avoid Oracle subquery factoring:

Say we want to create a report of the total counts of tables and indexes in the database. This is a very useful report for the DBA to ensure that no new objects have migrated into the production environment. We could also see the total bytes for all tables and indexes and the size change over the past week.

This would be a very sophisticated DBA report, and one that could run for many hours without the use of temporary tables because of Oracle's use of Oracle subquery factoring. However, with the use of temporary tables, the table and index counts can be summarized and saved in the temp tables to avoid Oracle subquery factoring. We also use the same technique to sum the number of bytes in all tables and indexes into temporary tables, and then quickly interrogate the summary tables for total sizes of our database, once again avoiding Oracle subquery factoring.

To avoid Oracle subquery factoring you could create a report generation SQL script. The code would compute the date ranges and computes the total table and index counts and bytes. An example is shown below:

The above example would allow you to avoid the slow down caused by Oracle subquery factoring.

You can also parallelize the full-scan, if you determine that the FTS is legitimate, this would reduce the impact of Oracle subquery factoring.

If I were tuning this, I would test it with plain CTAS and force index usage.


 

 

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