[_e9sw]

The `sum(1)` and `order by...limit` approach really isn't the best way to build the funnel. And if you take another approach then this could have easily been built with normal left joins.

Also, you should probably show some explain plans before making this claim: "Without lateral joins, we would need to resort to PL/pgSQL to do this analysis. Or, if our data set were small, we could get away with complex, inefficient queries."

Here's a comparison of the explain plan from your query without the `sum(1)` and `order by...limit` business and a query using only left joins (no use of lateral): [link redacted]. Note, I ran this against an empty copy of your exact table (no data, no statistics). However, the explain plans are the same.

My understanding is that lateral was really meant for set returning functions like generate_series as others have already mentioned.

Edit: I should mention I know you were just trying to demonstrate how lateral works and that it is always good to see people writing about new Postgres features!

[ajw0100]

Looking at this a little further, the outer nested loop could cause issues in the left join vs. left join lateral version, depending on how many use_demo events there are in the week following the user's first view_homepage event. I added another query that uses CTEs which allows for intermediate aggregation which should make the size of the nested loops similar between both versions. However, I wouldn't be surprised if the CTEs take more memory than the lateral joins because CTEs are basically temp tables that only last for the duration of the query. Lateral may indeed be the best option but ideally I would populate this table with real data, gather statistics, and then run explain analyze on each query.