Question

In postgis, is the ST_GeomFromText call very expensive? I ask mostly because I have a frequently called query that attempts to find the point that is nearest another point that matches some criteria, and which is also within a certain distance of that other point, and the way I currently wrote it, it’s doing the same ST_GeomFromText twice:

 $findNearIDMatchStmt = $postconn->prepare(     'SELECT     internalid ' .     'FROM       waypoint ' .     'WHERE      id = ? AND ' .     '           category = ? AND '.     '           (b.category in (1, 3) OR type like ?) AND '.     '           ST_DWithin(point, ST_GeomFromText(?,' . SRID .     '           ),'.  SMALL_EPSILON . ') ' .     '           ORDER BY ST_Distance(point, ST_GeomFromText(?,', SRID .     '           )) ' .     '           LIMIT 1'); 

Is there a better way to re-write this?

Slightly OT: In the preview screen, all my underscores are being rendered as & # 9 5 ; – I hope that’s not going to show up that way in the post.

Answer 1

I don’t believe ST_GeomFromText() is particularly expensive, although in the past I’ve optimized PostGIS queries by creating a function, declaring a variable and then assigning the result of ST_GeomFromText to the variable.

Have you tried checking the execution plan for you query with a variety of different parameters because that should give you a definite idea of which bits of the query are taking the time?

I’m guessing most of the execution time will be in the calls to ST_DWithin() and ST_Distance(), although if the id and category columns aren’t indexed then it might be doing some interesting table scanning.