Define a strategy to ensure stats are collected during topology building

[strk]

It is very likely that the queries used during topology building are inefficient due to lack of statistics enabling use of indexes.

Statistics are not updated until transaction commit, so this is a problem for queries like:

UPDATE mycoverage SET topogeom = toTopoGeom(geom);

This ticket is to define a strategy to run ANALYZE during these kind of operations, in a somewhat automatic but not invasive way.

[strk]

So one idea is for functions like ST_AddEdgeNewFaces, ST_AddEdgeModFaces, ST_AddEdge and the like to all use a common "edge adder" function which would keep track of how many primitives are added and run ANALYZE <table> after every <X> additions in <table>. Since every primitive table has an id, driven by a sequence, it could be relatively inexpensive to do, using a modulo operation.

[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.

[robe]

Milestone renamed