Why does using 'or' in a join in Vertica SQL cause significant performance issues?

Data and Databases
1

I’m encountering a performance problem with a SQL query in Vertica. Initially, I run this query to count distinct IDs:

SELECT COUNT(DISTINCT tab1.id)
FROM tab1 JOIN tab2 ON tab1.email = tab2.email_a;

However, when I modify it to include an ‘or’ condition as shown below, the execution seems to slow down drastically:

SELECT COUNT(DISTINCT tab1.id)
FROM tab1 JOIN tab2 ON tab1.email = tab2.email_a OR tab1.email = tab2.email_b;

While I understand that I can construct the query with two separate joins, I’m curious about what specifically occurs within Vertica’s processing that causes such a degradation in efficiency.

2

maybe it’s tht OR conditions lead to a Cartesian product affect in joint process. Vertica optimizes linear execution paths better than branched ones. splitting queries or maybe indexing the columns involved might improve the performance if the schema allows.

3

It might be due to the optimizer not handling OR effectively. It’s often more complex than straighfoward conditions, leading to scanning more records. Vertica’s engine may have to explore broader join possibilities which can bloat the query execution time. Hope this helps a bit!

4

Vertica’s columnar architecture is optimized for certain operations, and when you introduce an ‘OR’ in your join condition, each row may potentially be evaluated twice. This not only increases the complexity but also hinders the use of parallel execution. Additionally, query planning might not fully optimize for ‘OR’ conditions, resulting in redundant computation paths. Reorganizing the conditions into separate queries or using UNION to combine results can sometimes alleviate the bottleneck and improve performance.