SAP HANA计算视图中JOINS的性能

Question

For Example: I have 4 columns (A,B,C,D).

I thought that instead of connecting each and every column in join I should make a concatenated column in both projection(CA_CONCAT-> A+B+C+D) and make a join on this, Just to check on which method performance is better.

It was working faster earlier but in few CV's this method is slower sometimes, especially at time of filtering!

Can any one suggest which is an efficient method?

Answer 1

I don't think the JOIN conditions with concatenated fields will work better in performance.

Although we say in general there is not a need for index on column tables on HANA database, the column tables have a structure that works with an index on every column. So if you concatenate 4 columns and produce a new calculated field, first you loose the option to use these index on 4 columns and the corresponding joining columns

I did not check the execution plan, but it will probably make a full scan on these columns

In fact I'm surprised you have mentioned that it worked faster, and experienced problems only on a few

Because concatenation or applying a function on a database column is even only by itself a workload over the SELECT process. It might include implicit type cast operation, which might bring additional workload more than expected

Answer 2

First I would suggest considering setting your table to column store and check the new performance.

After that I would suggest to separate the JOIN to multiple JOINs if you are using OR condition in your join.

Third, INNER JOIN will give you better performance compare to LEFT JOIN or LEFT OUTER JOIN.

Another thing about JOINs and performance, you better use them on PRIMARY KEYS and not on each column.

Answer 3

For me, both the time join with multiple fields is performing faster than join with concatenated fields. For filtering scenario, planviz shows when I join with multiple fields, filter gets pushed down to both the tables. On the other hand, when I join with concatenated field only one table gets filtered.

However, if you put filter on both the fields (like PRODUCT from Tab1 and MATERIAL from Tab2), then you can push the filter down to both the tables.

Like:

Select * from CalculationView where PRODUCT = 'A' and MATERIAL = 'A'