quadratic (?) complexity when composing fragments

[fdietze]

Concatenating 10k Fragments 10 times is much faster than concatenating 100k fragments one time. This suggests a quadratic (or nonlinear) runtime complexity.

Reproduction in ammonite (Scala 2.13.8):

import $ivy.`org.tpolecat::skunk-core:0.2.3`; import skunk._; import skunk.implicits._

def measure(code: => Any) = {val start = System.nanoTime; code; println(f"${(System.nanoTime - start)/1000000000.0}%.2fs")}

measure { for(i <- 0 until 10) Array.fill(10000)(sql"x").foldLeft(sql"")((a,b) => sql"$a$b") }
// 18s
// is much faster than
measure { Array.fill(100000)(sql"x").foldLeft(sql"")((a,b) => sql"$a$b") }
// 213s

It could be caused by repeated concatenation of a growing List.