[postgis-tickets] [PostGIS] #2998: Define a strategy to ensure stats are collected during topology building

[PostGIS]

#2998: Define a strategy to ensure stats are collected during topology building
-------------------------+--------------------------------------------------
 Reporter:  strk         |       Owner:  strk          
     Type:  enhancement  |      Status:  new           
 Priority:  medium       |   Milestone:  PostGIS Future
Component:  topology     |     Version:  trunk         
 Keywords:               |  
-------------------------+--------------------------------------------------

Comment(by strk):

 Note that it is not necessarely true that indexes won't be used due to
 lack of stats, as the estimator makes some guesses on itself, and the
 invoked type-specific selectivity estimators may also behave in unexpected
 ways.

 In this example, passing 99 geometries to toTopoGeom in a single statement
 resulted in these operations on the "edge_data" table:

 {{{
 =# select * from pg_stat_all_tables where relname = 'edge_data' and
 schemaname = 'topo_ulfareale';
 relid             | 16228229
 schemaname        | topo_ulfareale
 relname           | edge_data
 seq_scan          | 417
 seq_tup_read      | 28976
 idx_scan          | 3438
 idx_tup_fetch     | 3170
 n_tup_ins         | 175
 n_tup_upd         | 365
 n_tup_del         | 0
 n_tup_hot_upd     | 65
 n_live_tup        | 175
 n_dead_tup        | 365
 last_vacuum       |
 last_autovacuum   |
 last_analyze      |
 last_autoanalyze  |
 vacuum_count      | 0
 autovacuum_count  | 0
 analyze_count     | 0
 autoanalyze_count | 0
 }}}

 As you can see there was no analyze ever run, still there have been ~3.5k
 index scans and "only" 417 sequencial scans. For just 99 inputs it's much
 over too many scans in general, but shows that index scans still happen.

-- 
Ticket URL: <http://trac.osgeo.org/postgis/ticket/2998#comment:2>
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