Towards the end of last year I used a query with a couple of “constant” subqueries as a focal point for a blog note on reading parallel execution plans. One of the comments on that note raised a question about cardinality estimates and, coincidentally, I received an email about the cost calculations for a similar query a few days later.
Unfortunately there are all sorts of anomalies, special cases, and changes that show up across versions when subqueries come into play – it’s only in recent versions of 11.2, for example, that a very simple example I’ve got of three equivalent statements that produce the same execution plan report the same costs and cardinality. (The queries are: table with IN subquery, table with EXISTS subquery, table joined to “manually unnested” subquery – the three plans take the unnested subquery shape.)
