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
--with-llvm.
See src/backend/jit/README for further details.
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 (see Section 75.6.1) 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; see Chapter 40. In fact the built-in objects are implemented using nearly the same mechanisms. This extensibility implies some overhead, for example due to function calls (see Section 40.3). 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.