Surafel, On Wed, Jul 17, 2019 at 3:45 AM Surafel Temesgen <surafel3...@gmail.com> wrote:
> > Hi Ryan, > On Tue, Jul 9, 2019 at 4:13 PM Ryan Lambert <r...@rustprooflabs.com> > wrote: > >> >> "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. Thanks, Ryan >
