MonetDB Query Plan

Question

I have a few queries that I am running and I would like to view some sort of query plan for a given query. When I add "explain" before the query, I get a long (~4,000 lines) result that is not possible to interpret.

Answer 1

The MAL plan exposes all parallel activity needed to solve the query. Each line is a relational algebra operator or catalog action.

You might also use PLAN to get an idea of the output of the SQL optimizer.

Answer 2

Each part in the physical execution plan that'll be executed in parallel is repeated the same number of times as the number of cores you have in the result of EXPLAIN. That's why EXPLAIN can sometimes produce a huge MAL plan.

If you just want to have an idea of how are query is handled, you can force MonetDB to generate a sequential MAL plan, then at least, you get rid of the repetitions. For this, you can change the default optimiser pipe line to, e.g., 'sequential_pipe'. This can be done both in a client (it works then only for this client session), or in a server (it works then for the whole server session). For more information: https://www.monetdb.org/Documentation/Cookbooks/SQLrecipes/OptimizerPipelines