Show heap fetches as percent %

[pgiraud]

Show heap fetches as percent %

[Krysztophe]

The % of blocks is perhaps more important, but you can't have that.

The threshold should be rather low. A rather small proportion is already the sign that you need a VACUUM.

I hope to find time to test soon.

[Krysztophe]

I confirm: a few percent of heap fetches is already a disaster.

Examples (in the worst case: old computer, default configuration, not enough RAM, slow HDD) :

• clean table after an INSERT, vacuumed:

                                                     QUERY PLAN                                                      
═════════════════════════════════════════════════════════════════════════════════════════════════════════════════════
 Index Only Scan using demoheapfetches_i_j_idx on demoheapfetches (actual time=0.984..1477.217 rows=1000000 loops=1)
   Heap Fetches: 0
   Buffers: shared hit=601242 read=2735
 Planning:
   Buffers: shared hit=6
 Planning Time: 0.273 ms
 Execution Time: 1751.430 ms

• 10% new lines, autovacuum is lagging: execution time +100 %

                                                     QUERY PLAN                                                      
═════════════════════════════════════════════════════════════════════════════════════════════════════════════════════
 Index Only Scan using demoheapfetches_i_j_idx on demoheapfetches (actual time=0.066..2935.111 rows=1100000 loops=1)
   Heap Fetches: 100001
   Buffers: shared hit=745883 read=2486
 Planning Time: 0.186 ms
 Execution Time: 3365.290 ms

• 30% new lines, autovacuum still lagging: execution time × 78 !!

                                                      QUERY PLAN                                                       
═══════════════════════════════════════════════════════════════════════════════════════════════════════════════════════
 Index Only Scan using demoheapfetches_i_j_idx on demoheapfetches (actual time=0.069..136461.243 rows=1300000 loops=1)
   Heap Fetches: 300001
   Buffers: shared hit=894764 read=112141
 Planning Time: 0.205 ms
 Execution Time: 136901.507 ms

• after a vacuum: almost back to normal

                                                     QUERY PLAN                                                      
═════════════════════════════════════════════════════════════════════════════════════════════════════════════════════
 Index Only Scan using demoheapfetches_i_j_idx on demoheapfetches (actual time=0.112..2040.199 rows=1300000 loops=1)
   Heap Fetches: 0
   Buffers: shared hit=811331 read=2857
 Planning:
   Buffers: shared hit=15 read=2
 Planning Time: 349.595 ms
 Execution Time: 2405.755 ms