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formatting

Pete 2016-06-28 16:15:34 +01:00
parent 87e67e78da
commit d778967a57
1 changed files with 7 additions and 2 deletions
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OptimizeRenderingWithPostGIS.md

@ -214,13 +214,16 @@ If there is any active connection Postgresql will wait until it is closed, so if
## Use an extent parameter ## Use an extent parameter
If an extent parameter is not set, mapnik will perform a query like this... If an extent parameter is not set, mapnik will perform a query like this...
`SELECT ST_XMin(ext),ST_YMin(ext),ST_XMax(ext),ST_YMax(ext)` ```
`FROM (SELECT ST_Extent(geom) as ext from planet_osm_line) as tmp` SELECT ST_XMin(ext),ST_YMin(ext),ST_XMax(ext),ST_YMax(ext)
FROM (SELECT ST_Extent(geom) as ext from planet_osm_line) as tmp
```
...which requires PostGIS to walk the entire result set of the queried table each time the DataSource is used for the first time in a rendering session. There are three parameters available for use ...which requires PostGIS to walk the entire result set of the queried table each time the DataSource is used for the first time in a rendering session. There are three parameters available for use
### extent_from_subquery ### extent_from_subquery
E.g. `<Parameter name="extent_from_subquery">true</Parameter>` E.g. `<Parameter name="extent_from_subquery">true</Parameter>`
**Pros:** **Pros:**
* Precise estimate of extent * Precise estimate of extent
@ -237,6 +240,7 @@ E.g. `<Parameter name="extent_from_subquery">true</Parameter>`
### extent ### extent
E.g. `<Parameter name="extent">-20037508,-19929239,20037508,19929239</Parameter>` E.g. `<Parameter name="extent">-20037508,-19929239,20037508,19929239</Parameter>`
**Pros:** **Pros:**
* No database overhead * No database overhead
@ -256,6 +260,7 @@ E.g. `<Parameter name="extent">-20037508,-19929239,20037508,19929239</Parameter>
### estimate_extent ### estimate_extent
E.g. `<Parameter name="estimate_extent">true</Parameter>` E.g. `<Parameter name="estimate_extent">true</Parameter>`
**Pros:** **Pros:**
* Faster than not setting any extent parameters; significantly for large result sets * Faster than not setting any extent parameters; significantly for large result sets