[Tile-serving] [openstreetmap/mod_tile] memory consumption (#181)

[duerk-de]

In my case there are four tables with an index for each of them, like osm2pgsql creates. A geometry is indexed using postgis plugin.

I did not dig into detail of postgis or postgresql caching mechanisms but I made a simple test using logged queries from tile rendering. I wrote a php script issuing this queries with the option of closing and reopening the connection in between.

During querying the database a postgresql process increased memory consumption. The memory consumtion of the php script did not change at all. After closing the connection the postgresql process vanished and all memory was freed.

I did not see any memory increase on client side which is in contradiction to the comments here and to what I experienced after starting the rendering session. But after some time of rendering, where low zoom levels with large query result sets are completely rendered, there seems to be low memory consumption increase on client (renderd) side but on server side.

For the record some rough data:
- memory increase: 10 GB per day
- database queries: 450000 per day
- unique database queries: 150000 per day

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