Help the planner with effective_cache_size
effective_cache_size tells the planner how much memory it can assume is available for caching. It allocates nothing, but raising it lowers the estimated cost of index-friendly plans, here from 10612 to 8240 for the same query.
Problem
What you're actually looking at
The symptom as it shows up on a real server.
effective_cache_size does not reserve memory; it is a hint about how much OS+shared cache the planner can assume when costing repeated index access. Set too low, the planner overestimates how often index pages must be re-read from disk and shies away from nested-loop/index plans.
Meridian's effective_cache_size is left at a conservative default, so the planner costs its index-driven join pessimistically. Raising it to reflect actual RAM lowers the estimated cost of the same plan.
Simple terms
effective_cache_size does not reserve any memory, it is only a hint telling the planner how much RAM it can assume is available for caching data. Set it too low and the planner assumes index pages will constantly have to be re-read from disk, so it avoids index-driven plans. Raising it to reflect the machine's real memory lowers the estimated cost of those plans, so the planner stops being needlessly pessimistic.
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