Arnaud Lesauvage <thewild@freesurf.fr> writes:
> When I join these two tables, the 2-column index of the first table is
> not used.
> Why does the query planner think that this plan is better ?
Hm, is gid by itself nearly unique in these tables? If so, the merge
join would get only marginally more efficient by using both columns as
merge conditions. Heikki's probably right to guess that the planner
thinks it's better to use the smaller index.
However, if there are lots of duplicate gids, then it ought to have
preferred the two-column merge ...
regards, tom lane
