The GEQO module approaches the query optimization problem as though it were the well-known traveling salesman problem (TSP). Possible query plans are encoded as integer strings. Each string represents the join order from one relation of the query to the next. For example, the join tree
/\ /\ 2 /\ 3 4 1
is encoded by the integer string '4-1-3-2', which means, first join relation '4' and '1', then '3', and then '2', where 1, 2, 3, 4 are relation IDs within the PostgreSQL optimizer.
Parts of the GEQO module are adapted from D. Whitley's Genitor algorithm.
Specific characteristics of the GEQO implementation in PostgreSQL are:
Usage of a steady state GA (replacement of the least fit individuals in a population, not whole-generational replacement) allows fast convergence towards improved query plans. This is essential for query handling with reasonable time;
Usage of edge recombination crossover which is especially suited to keep edge losses low for the solution of the TSP by means of a GA;
Mutation as genetic operator is deprecated so that no repair mechanisms are needed to generate legal TSP tours.
The GEQO module allows the PostgreSQL query optimizer to support large join queries effectively through non-exhaustive search.
Work is still needed to improve the genetic algorithm parameter settings. In file src/backend/optimizer/geqo/geqo_main.c, routines gimme_pool_size and gimme_number_generations, we have to find a compromise for the parameter settings to satisfy two competing demands:
Optimality of the query plan
Computing time
At a more basic level, it is not clear that solving query optimization with a GA algorithm designed for TSP is appropriate. In the TSP case, the cost associated with any substring (partial tour) is independent of the rest of the tour, but this is certainly not true for query optimization. Thus it is questionable whether edge recombination crossover is the most effective mutation procedure.
