Define a guideline for function cost

[strk]

A guideline could help with defining the cost of each function signature, to help with cases like #2895 and #2896.

Such guideline could written in the top-level .sql.in files of each component (core, raster, topology, …).

[strk]

Example guideline for rasters:

-- COST assignment guideline (TODO):                                            
--                                                                              
-- By default C functions get assigned a cost of 1 while (plpg)sql functions    
-- get assigned a cost of 100. Value expressed in "cpu_operator_cost" units.    
-- See http://www.postgresql.org/docs/9.1/static/runtime-config-query.html#GUC-CPU-OPERATOR-COST
--                                                                              
-- The following schema can be used as a guideline to set cost of each          
-- function. The multiplier would apply to the language based default           
-- (1 or 100) and to the number of raster arguments.                            
--                                                                              
-- For example a multiplier of 500 for a C function taking 2 rasters            
-- and performing operations on each of their pixel would have a cost           
-- of 1000.                                                                     
--                                                                              
--  o Only detoast raster header                                                
--    x 1                                                                       
--                                                                              
--  o Detoast whole raster                                                      
--    x 100                                                                     
--                                                                              
--  o Scan each raster pixel                                                    
--    x 300                                                                     
--                                                                              
--  o Perform operations on each raster pixel                                   
--    x 500                                  

[strk]

The guideline example above is completely off, though. Here are some real numbers:

          noop : Total runtime: 19523.492 ms (full deserialize/serialize)       
   st_envelope : Total runtime: 18411.870 ms (header slice)                     
 st_convexhull : Total runtime: 18104.542 ms                                    
         plain : Total runtime: 51.341 ms (???) 

The times were taken running queries against a ~180k rows raster coverage. The "plain" is a "SELECT rast FROM …". The "noop" is a "SELECT postgis_noop(rast) FROM …".

The difference between "plain" and "noop" is impressive!!

[strk]

If I change the postgis_noop implementation to simply return the input datum it takes 66ms (vs. ~50ms of the plain timing). Having it simply do the DETOAST_DATUM makes it jump again to 18149.952 ms

It turns out EXPLAIN ANALYZE does not run canonical output functions at all, so there's no detoasting performed in a "SELECT rast FROM …"

[strk]

Alright so in the light of the new findings, sliced detoasting (header only) and full detoasting do not make such a big difference. It is detoasting itself that hurts. Guidelines should then not take that part much in consideration, other than a COST offset common to each and every function.

Besides, I suspect that the planner should be considering cost of detoasting on itself, as the same function could be invoked with the input already detoasted…

[strk]

17:37 < RhodiumToad> the planner doesn't take it into account. 17:38 < RhodiumToad> currently, functions that may be highly expensive due to detoasting (to_tsvector being a notorious example) often need to be assigned large costs to avoid bad plans

[dustymugs]

The numbers you're seeing generally match what I've seen with in-db rasters on production servers. For basic ops, the detoast is the costliest.

[strk]

The question is should we consider detoasting costs in the function definition or shouldn't we ? I'd think the planner should consider them itself as we cannot know its cost at definition time for two reasons:

1. It depends on input (toast) size
2. It depends on whether the input gets to us already detoasted

[pramsey]

From the commentary, it sounds like this one is not as "straightforward" as one might hope, if input data size is the primary determinant of cost. Maybe close.

[robe]

Milestone renamed