Nicolas Seinlet <nicolas@xxxxxxxxxxx> writes: > I'm trying to understand this behaviour and the limits of CTE, when they reach the limits, when they cannot receive parameters from a caller, ... I'm running a query on PostgreSQL 10 with a cte. the query runs in ~ 10 seconds. If I add a second CTE with the same query as the previous one and select * from second_cte as query, it now runs in ~ 10 minutes. > oversimplified example: > 10 seconds version: > | WITH cte1 AS (SELECT x,y,z FROM table) SELECT row_number() over(),x,y,z FROM cte1 WHERE x=32; > 10 minutes version: > | WITH cte1 AS (SELECT x,y,z FROM table), cte2 AS (SELECT row_number() over(),x,y,z FROM cte1) SELECT * FROM cte2 WHERE x=32; [ shrug... ] You are asking for two different computations, and the second one is far more expensive. In the first case, the WHERE x=32 clause is applied before the window function, so we can (indeed must) filter out all rows not having x=32 before doing the window function. In the second case, WHERE x=32 is applied above/after the window function. We cannot push down the WHERE to before the window function. (In this case, filtering beforehand would obviously change the results of row_number, but in general we don't know enough about window function behavior to risk such changes.) So row_number has to be computed over the entire contents of the "table", and that's not cheap. It does surprise me a bit that row_number is quite *that* expensive, but if you are expecting equivalent results from these two queries, you're simply wrong. regards, tom lane
