32.2. When to JIT?

    To determine whether JIT compilation should be used, the total estimated cost of a query (see Chapter 70 and ) is used. The estimated cost of the query will be compared with the setting of jit_above_cost. If the cost is higher, JIT compilation will be performed. Two further decisions are then needed. Firstly, if the estimated cost is more than the setting of , short functions and operators used in the query will be inlined. Secondly, if the estimated cost is more than the setting of jit_optimize_above_cost, expensive optimizations are applied to improve the generated code. Each of these options increases the JIT compilation overhead, but can reduce query execution time considerably.

    If is set to , or if no JIT implementation is available (for example because the server was compiled without --with-llvm), JIT will not be performed, even if it would be beneficial based on the above criteria. Setting jit to off has effects at both plan and execution time.

    Given the cost of the plan, it is entirely reasonable that no JIT was used; the cost of JIT would have been bigger than the potential savings. Adjusting the cost limits will lead to JIT use:

    1. =# SET jit_above_cost = 10;
    2. SET
    4. -------------------------------------------------------------------------------------------------------------
    5.  Aggregate  (cost=16.27..16.29 rows=1 width=8) (actual time=6.049..6.049 rows=1 loops=1)
    6.    ->  Seq Scan on pg_class  (cost=0.00..15.42 rows=342 width=4) (actual time=0.019..0.052 rows=356 loops=1)
    8.    Functions: 3
    9.    Options: Inlining false, Optimization false, Expressions true, Deforming true
    10.    Timing: Generation 1.259 ms, Inlining 0.000 ms, Optimization 0.797 ms, Emission 5.048 ms, Total 7.104 ms