Based on the documentation, I expected the first planned query text to be: `SELECT 10 AS data`, since it should be a custom plan with substituted values. However, the query text always contains a parameter symbol: `SELECT $1 AS data`.
A query plan is not the same as the query text. While the executed plan can be generic or custom the query text is constant.
Thanks for drawing my attention to the difference between the query text and query plan.
If you want to see the difference between a generic and a custom plan you need to comprise a query that would actually have different custom and generic plans. Queries that don’t involve tables, indexes, or joins don’t have any choices to make with respect to how they are executed.
After reading Laurenz's reply, I experimented again with `PREPARE basic(int) AS SELECT $1 AS number;" and the query plan logged in the log file (by auto_explain) does indeed show "Output: $1" on the sixth EXPLAIN EXECUTE! Sorry I missed this earlier. (The EXPLAIN EXECUTE output itself does not contain an "Output:" line, so I didn't notice this lack of parameter substitution in psql.)
I appreciate your point about query plans which may not involve choices; thank you for mentioning it.
