Update documentation to include instructions for spatial indexing of points in srid 3857

[chriszrc]

Given that all the point intersection queries are executed with srid of 3857:

clusterbuster/lib/queries/base.ts

Lines 16 to 24 in a47d246

	SELECT
	${geometry} AS center,
	1 AS size,
	0 AS clusterNo,
	${maxZoomLevel + 1} AS expansionZoom${attributes}
	FROM ${table}
	WHERE
	ST_Intersects(TileBBox(${z}, ${x}, ${y}, 3857), ST_Transform(${geometry}, 3857))
	${query.length > 0 ? `AND ${query.join(' AND ')}` : ''}

A standard spatial index on the point geometry column won't be used (unless maybe the points are stored in 3857, but most I suspect are probably 4326). However regardless of srid on the geometry column, we can always add an expression (functional) index:

CREATE INDEX mytable_geom_3857_idx on mytable USING gist (ST_Transform(geom, 3857));

Which will of course speed up ST_INTERSECTS queries for all the tile requests.

PS - Was https://github.com/datalanche/node-pg-format or similar considered to more safely create parameterized queries?

[cosmin-petrescu]

Hello @chriszrc . Do you want to create a PR for this issue. We are more than happy to review it and merge it.