32.2. When to JIT?

32.2. When to JIT?
Prev	Up	Chapter 32. Just-in-Time Compilation (JIT)	Home	Next

+ JIT compilation is beneficial primarily for long-running + CPU-bound queries. Frequently these will be analytical queries. For short + queries the added overhead of performing JIT compilation + will often be higher than the time it can save. +

+ To determine whether JIT compilation should be used, + the total estimated cost of a query (see + Chapter 72 and + Section 20.7.2) 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 jit_inline_above_cost, 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. +

+ These cost-based decisions will be made at plan time, not execution + time. This means that when prepared statements are in use, and a generic + plan is used (see PREPARE), the values of the + configuration parameters in effect at prepare time control the decisions, + not the settings at execution time. +

Note

+ If jit is set to off, 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. +

+ EXPLAIN can be used to see whether + JIT is used or not. As an example, here is a query that + is not using JIT: +

+=# EXPLAIN ANALYZE SELECT SUM(relpages) FROM pg_class;
+                                                 QUERY PLAN
+-------------------------------------------------------------------------------------------------------------
+ Aggregate  (cost=16.27..16.29 rows=1 width=8) (actual time=0.303..0.303 rows=1 loops=1)
+   ->  Seq Scan on pg_class  (cost=0.00..15.42 rows=342 width=4) (actual time=0.017..0.111 rows=356 loops=1)
+ Planning Time: 0.116 ms
+ Execution Time: 0.365 ms
+(4 rows)
+

+ 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: +

+=# SET jit_above_cost = 10;
+SET
+=# EXPLAIN ANALYZE SELECT SUM(relpages) FROM pg_class;
+                                                 QUERY PLAN
+-------------------------------------------------------------------------------------------------------------
+ Aggregate  (cost=16.27..16.29 rows=1 width=8) (actual time=6.049..6.049 rows=1 loops=1)
+   ->  Seq Scan on pg_class  (cost=0.00..15.42 rows=342 width=4) (actual time=0.019..0.052 rows=356 loops=1)
+ Planning Time: 0.133 ms
+ JIT:
+   Functions: 3
+   Options: Inlining false, Optimization false, Expressions true, Deforming true
+   Timing: Generation 1.259 ms, Inlining 0.000 ms, Optimization 0.797 ms, Emission 5.048 ms, Total 7.104 ms
+ Execution Time: 7.416 ms
+

+ As visible here, JIT was used, but inlining and + expensive optimization were not. If jit_inline_above_cost or jit_optimize_above_cost were also lowered, + that would change. +

Prev	Up	Next
32.1. What Is JIT compilation?	Home	32.3. Configuration