This chapter explains what just-in-time compilation is, and how it can be configured in PostgreSQL.
What Is JIT compilation?
Just-in-Time (JIT) compilation is the process of turning some form of interpreted program evaluation into a native program, and doing so at run time. For example, instead of using general-purpose code that can evaluate arbitrary SQL expressions to evaluate a particular SQL predicate like
WHERE a.col = 3, it is possible to generate a function that is specific to that expression and can be natively executed by the CPU, yielding a speedup.
PostgreSQL has builtin support to perform JIT compilation using LLVM when PostgreSQL is built with
src/backend/jit/README for further details.
JIT Accelerated Operations
Currently PostgreSQL’s JIT implementation has support for accelerating expression evaluation and tuple deforming. Several other operations could be accelerated in the future.
Expression evaluation is used to evaluate
WHERE clauses, target lists, aggregates and projections. It can be accelerated by generating code specific to each case.
Tuple deforming is the process of transforming an on-disk tuple into its in-memory representation. It can be accelerated by creating a function specific to the table layout and the number of columns to be extracted.
PostgreSQL is very extensible and allows new data types, functions, operators and other database objects to be defined. In fact the built-in objects are implemented using nearly the same mechanisms. This extensibility implies some overhead, for example due to function calls. To reduce that overhead, JIT compilation can inline the bodies of small functions into the expressions using them. That allows a significant percentage of the overhead to be optimized away.
LLVM has support for optimizing generated code. Some of the optimizations are cheap enough to be performed whenever JIT is used, while others are only beneficial for longer-running queries. See https://llvm.org/docs/Passes.html#transform-passes for more details about optimizations.
When to JIT?
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 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.
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;
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
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;
=# EXPLAIN ANALYZE SELECT SUM(relpages) FROM pg_class;
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
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.
The configuration variable jit determines whether JIT compilation is enabled or disabled. If it is enabled, the configuration variables jit_above_cost, jit_inline_above_cost, and jit_optimize_above_cost determine whether JIT compilation is performed for a query, and how much effort is spent doing so.
jit_provider determines which JIT implementation is used. It is rarely required to be changed.
For development and debugging purposes a few additional configuration parameters exist.
Inlining Support for Extensions
PostgreSQL’s JIT implementation can inline the bodies of functions of types
internal, as well as operators based on such functions. To do so for functions in extensions, the definitions of those functions need to be made available. When using PGXS to build an extension against a server that has been compiled with LLVM JIT support, the relevant files will be built and installed automatically.
The relevant files have to be installed into
$pkglibdir/bitcode/$extension/ and a summary of them into
$pkglibdir is the directory returned by
pg_config --pkglibdir and
$extension is the base name of the extension’s shared library.
Pluggable JIT Providers
PostgreSQL provides a JIT implementation based on LLVM. The interface to the JIT provider is pluggable and the provider can be changed without recompiling (although currently, the build process only provides inlining support data for LLVM). The active provider is chosen via the setting jit_provider.
JIT Provider Interface
A JIT provider is loaded by dynamically loading the named shared library. The normal library search path is used to locate the library. To provide the required JIT provider callbacks and to indicate that the library is actually a JIT provider, it needs to provide a C function named
_PG_jit_provider_init. This function is passed a struct that needs to be filled with the callback function pointers for individual actions:
extern void _PG_jit_provider_init(JitProviderCallbacks *cb);