Question

Which option is better:

1. Writing a very complex query having large number of joins, or
2. Writing 2 queries one after the other, applying the obtained result set of the processed query on other.

Answer 1

When I was a junior DB person I once worked for a year in a marketing dept where I had so much free time I did each task 2 or 3 different ways. I made a habit of writing one mega-select that grabbed everything in one go and comparing it to a script that built interim tables of selected primary keys and then once I had the correct keys went and got the data values.

In almost every case the second method was faster. the cases where it wasn’t were when dealing with a small number of small tables. Where it was most noticeably faster was of course large tables and multiple joins.

I got into the habit of select the required primary keys from tableA, select the required primary keys from tableB, etc. Join them and select the final set of primary keys. Use the selected primary keys to go back to the tables and get the data values.

As a DBA I now understand that this method resulted in less purging of the data cache and played nicer with others using the DB (as mentioned by Amir Raminfar).

It does however require the use of temporary tables which some places / DBA don’t like (unfairly in my mind)