Make use of the index while reading the config table

[gleu]

Performance are really bad without :) See https://github.com/dalibo/pg_query_settings/wiki/Benchmarks

[tilkow]

Hi Guillaume,

Here is a modest test with attempt to use SPI.

[gleu]

I did some tests last night. It seems to be working. The index is used, perfs don't move a bit if you have 1 to 500 configurations in the table. But, and this is a big one, performance are really bad. We have at least 30% less TPS. This is even worst than the 500-config test I did when we were scanning the whole table. I currently have no idea why, and no time to check. We'll have to understand what did go wrong. But at least, you finally found how to use the index, with is great. Kudos!