Question

I am working on a join condition between 2 tables where one of the columns to match on is a concatentation of values. I need to join columnA from tableA to the first 2 characters of columnB from tableB.

I have developed 2 different statements to handle this and I have tried to analyze the performance of each method.

Method 1:

ON tB.columnB   like  tA.columnA || '%'

Method 2:

ON substr(tB.columnB,1,2) = tA.columnA

The query execution plan has a lot less steps using Method 1 compared to Method 2, however, it looks like Method 2 executes much faster. Also, the execution plan shows a recommended index for Method 2 that could improve its performance.

I am running this on an IBM iSeries, though would be interested in answers in a general sense to learn more about sql query optimization.

Does it make sense that Method 2 would execute faster?

This SO question is similar, but it looks like no one provided any concrete answers to the performance difference of these approaches: T-SQL speed comparison between LEFT() vs. LIKE operator.

PS: The table design that requires this type of join is not something that I can get changed at this time. I realize having the fields separated which hold different types of data would be preferrable.

Answer 1

I found this reference in an IBM redbook related to SQL performance. It sounds like the SUBSTR scalar function can be handled in an optimized manner by an iSeries.

If you search for the first character and want to use the SQE instead
of the CQE, you can use the scalar function substring on the left sign
of the equal sign. If you have to search for additional characters in
the string, you can additionally use the scalar function POSSTR. By
splitting the LIKE predicate into several scalar function, you can
affect the query optimizer to use the SQE.

http://publib-b.boulder.ibm.com/abstracts/sg246654.html?Open