"It is possible for FETCH FIRST N PERCENT to create poorly performing query plans when the N supplied exceeds 50 percent. In these cases query execution can take an order of magnitude longer to execute than simply returning the full table. If performance is critical using an explicit row count for limiting is recommended."
I don’t understand how fetch first n percent functionality can be replaced with explicit row count limiting. There may be a way to do it in a client side but we can not be sure of its performance advantage
regards
Surafel
I was suggesting a warning in the documentation so users aren't caught unaware about the performance characteristics. My first version was very rough, how about adding this in doc/src/sgml/ref/select.sgml?
"Using <literal>PERCENT</literal> is best suited to returning single-digit percentages of the query's total row count."
The following paragraphs in that same section give suggestions and warnings regarding LIMIT and OFFSET usage, I think this is more in line with the wording of those existing warnings.
Other than that, we can rip the clause if it is 100%
You mean if PERCENT=100 it should short circuit and run the query normally? I like that.
That got me thinking, I didn't check what happens with PERCENT>100, I'll try to test that soon.