Parallel query and PLJava

Chapman Flack edited this page Oct 30, 2016 · 2 revisions

Parallel query and PL/Java

PL/Java 1.5.1 adds support for PostgreSQL 9.6, and with that comes the possibility of using PL/Java functions in parallel queries. Simple testing shows that this actually works; PL/Java functions can even be declared PARALLEL SAFE if they meet the requirements, and executed in the parallelized parts of queries.

However, this is a substantial change to conditions in which PL/Java was developed, so this wiki page is here to collect the notes that are likely to come with experience using this new capability. Such experience might include empirically-determined, good values for parallel_setup_cost, nonobvious cases where a function should not be declared RESTRICTED or SAFE, and so on.

Notes go here

Preview of new documentation

Until PL/Java 1.5.1 is released, here is a preview of the new section of the user's guide.

PL/Java in parallel query or background worker

With some restrictions, PL/Java can be used in parallel queries, from PostgreSQL 9.6, and in some background worker processes (as introduced in PostgreSQL 9.3, though 9.5 or later is needed for support in PL/Java).

Background worker processes

Because PL/Java requires access to a database containing the sqlj schema, PL/Java is only usable in a worker process that initializes a database connection, which must happen before the first use of any function that depends on PL/Java.

Parallel queries

Like any user-defined function, a PL/Java function can be annotated with a level of "parallel safety", UNSAFE by default.

When a function labeled UNSAFE is used in a query, the query cannot be parallelized at all. If a query contains a function labeled RESTRICTED, parts of the query may execute in parallel, but the part that calls the RESTRICTED function will be executed only in the lead process. A function labeled SAFE may be executed in every process participating in the query.

Parallel setup cost

PostgreSQL parallel query processing uses multiple operating-system processes, and these processes are new for each parallel query. If a PL/Java function is labeled PARALLEL SAFE and is pushed by the query planner to run in the parallel worker processes, each new process will start a Java virtual machine. The cost of doing so will reduce the expected advantage of parallel execution.

To inform the query planner of this trade-off, the value of the PostgreSQL configuration variable parallel_setup_cost should be increased. The startup cost can be minimized with attention to the PL/Java VM option recommendations, including class data sharing.

Limits on RESTRICTED/SAFE function behavior

There are stringent limits on what a function labeled RESTRICTED may do, and even more stringent limits on what may be done in a function labeled SAFE. The PostgreSQL manual describes the limits in the section Parallel Labeling for Functions and Aggregates.

While PostgreSQL does check for some inappropriate operations from a PARALLEL SAFE or RESTRICTED function, for the most part it relies on functions being labeled correctly. When in doubt, the conservative approach is to label a function UNSAFE, which can't go wrong. A function mistakenly labeled RESTRICTED or SAFE could produce unpredictable results.

Internal workings of PL/Java

While a given PL/Java function itself may clearly qualify as RESTRICTED or SAFE by inspection, there may still be cases where a forbidden operation results from the internal workings of PL/Java itself. This has not been seen in testing (simple parallel queries with RESTRICTED or SAFE PL/Java functions work fine), but to rule out the possibility would require a careful audit of PL/Java's code. Until then, it would be prudent for any application involving parallel query with RESTRICTED or SAFE PL/Java functions to be first tested in a non-production environment.

Further reading

README.parallel in the PostgreSQL source, for more detail on why parallel query works the way it does.

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