update non-indexed value is slow if some non-related index/fk are enabled

Update on public.t_acte_entite (cost=0.00..52924.83 rows=2758149 width=45) (actual time=120908.338..120908.338 rows=0 loops=1)
Buffers: shared hit=101546408 read=164106 dirtied=171996 written=9529
-> Seq Scan on public.t_acte_entite (cost=0.00..52924.83 rows=2758149 width=45) (actual time=0.028..1783.946 rows=3033768 loops=1)
Output: id_acte_entite, fk_acte, fk_rubrique, fk_client, fk_salarie, fk_cycldet, fk_planning, act_user_modified, fk_ssiad_visite, fk_ssiad_visite_planning, true, acte_realise, acte_realise_modifie, ctid
Filter: ((NOT t_acte_entite.acte_prevu) AND ((t_acte_entite.fk_ssiad_visite_planning IS NOT NULL) OR (t_acte_entite.fk_ssiad_visite IS NOT NULL)))
Rows Removed by Filter: 16115
Buffers: shared hit=10 read=22416 written=9441
Planning time: 0.133 ms
Execution time: 120908.387 ms

Explain with index disabled :

You can see that the Buffers: shared hit= drop from 101 Millions to 10Million without the indexes

But as the query plan only do a Sec Scan and don't acces an index I'm little surprised.

Same on postgres 13 :

//without explain to know how many update are done :

test=# update t_acte_entite set acte_prevu = true where (fk_ssiad_visite_planning is not null or fk_ssiad_visite is not null);
UPDATE 3049406

//First explain ( with index enabled )

test=# explain (analyse, COSTS, buffers, verbose, FORMAT JSON) update t_acte_entite set acte_prevu = true where (fk_ssiad_visite_planning is not null or fk_ssiad_visite is not null);
[
  {
    "Plan": {
      "Node Type": "ModifyTable",
      "Operation": "Update",
      "Parallel Aware": false,
      "Relation Name": "t_acte_entite",
      "Schema": "public",
      "Alias": "t_acte_entite",
      "Startup Cost": 0.00,
      "Total Cost": 87603.92,
      "Plan Rows": 2748711,
      "Plan Width": 46,
      "Actual Startup Time": 74072.048,
      "Actual Total Time": 74072.049,
      "Actual Rows": 0,
      "Actual Loops": 1,
      "Shared Hit Blocks": 89724572,
      "Shared Read Blocks": 115370,
      "Shared Dirtied Blocks": 101855,
      "Shared Written Blocks": 54351,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 0,
      "Temp Written Blocks": 0,
      "Plans": [
        {
          "Node Type": "Seq Scan",
          "Parent Relationship": "Member",
          "Parallel Aware": false,
          "Relation Name": "t_acte_entite",
          "Schema": "public",
          "Alias": "t_acte_entite",
          "Startup Cost": 0.00,
          "Total Cost": 87603.92,
          "Plan Rows": 2748711,
          "Plan Width": 46,
          "Actual Startup Time": 0.047,
          "Actual Total Time": 1040.544,
          "Actual Rows": 3049406,
          "Actual Loops": 1,
          "Output": ["id_acte_entite", "fk_acte", "fk_rubrique", "fk_client", "fk_salarie", "fk_cycldet", "fk_planning", "act_user_modified", "fk_ssiad_visite", "fk_ssiad_visite_planning", "true", "acte_realise", "acte_realise_modifie", "ctid"],
          "Filter": "((t_acte_entite.fk_ssiad_visite_planning IS NOT NULL) OR (t_acte_entite.fk_ssiad_visite IS NOT NULL))",
          "Rows Removed by Filter": 477,
          "Shared Hit Blocks": 4334,
          "Shared Read Blocks": 53032,
          "Shared Dirtied Blocks": 0,
          "Shared Written Blocks": 14709,
          "Local Hit Blocks": 0,
          "Local Read Blocks": 0,
          "Local Dirtied Blocks": 0,
          "Local Written Blocks": 0,
          "Temp Read Blocks": 0,
          "Temp Written Blocks": 0
        }
      ]
    },
    "Planning": {
      "Shared Hit Blocks": 94,
      "Shared Read Blocks": 12,
      "Shared Dirtied Blocks": 2,
      "Shared Written Blocks": 0,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 0,
      "Temp Written Blocks": 0
    },
    "Planning Time": 1.007,
    "Triggers": [
    ],
    "Execution Time": 74075.229
  }
]

//Second explain ( with index enabled )

test=# explain (analyse, COSTS, buffers, verbose, FORMAT JSON) update t_acte_entite set acte_prevu = true where (fk_ssiad_visite_planning is not null or fk_ssiad_visite is not null);
[
  {
    "Plan": {
      "Node Type": "ModifyTable",
      "Operation": "Update",
      "Parallel Aware": false,
      "Relation Name": "t_acte_entite",
      "Schema": "public",
      "Alias": "t_acte_entite",
      "Startup Cost": 0.00,
      "Total Cost": 91080.72,
      "Plan Rows": 2782747,
      "Plan Width": 46,
      "Actual Startup Time": 76641.220,
      "Actual Total Time": 76641.221,
      "Actual Rows": 0,
      "Actual Loops": 1,
      "Shared Hit Blocks": 94719974,
      "Shared Read Blocks": 122224,
      "Shared Dirtied Blocks": 104959,
      "Shared Written Blocks": 42452,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 0,
      "Temp Written Blocks": 0,
      "Plans": [
        {
          "Node Type": "Seq Scan",
          "Parent Relationship": "Member",
          "Parallel Aware": false,
          "Relation Name": "t_acte_entite",
          "Schema": "public",
          "Alias": "t_acte_entite",
          "Startup Cost": 0.00,
          "Total Cost": 91080.72,
          "Plan Rows": 2782747,
          "Plan Width": 46,
          "Actual Startup Time": 48.873,
          "Actual Total Time": 1128.069,
          "Actual Rows": 3049406,
          "Actual Loops": 1,
          "Output": ["id_acte_entite", "fk_acte", "fk_rubrique", "fk_client", "fk_salarie", "fk_cycldet", "fk_planning", "act_user_modified", "fk_ssiad_visite", "fk_ssiad_visite_planning", "true", "acte_realise", "acte_realise_modifie", "ctid"],
          "Filter": "((t_acte_entite.fk_ssiad_visite_planning IS NOT NULL) OR (t_acte_entite.fk_ssiad_visite IS NOT NULL))",
          "Rows Removed by Filter": 477,
          "Shared Hit Blocks": 4033,
          "Shared Read Blocks": 56444,
          "Shared Dirtied Blocks": 0,
          "Shared Written Blocks": 7256,
          "Local Hit Blocks": 0,
          "Local Read Blocks": 0,
          "Local Dirtied Blocks": 0,
          "Local Written Blocks": 0,
          "Temp Read Blocks": 0,
          "Temp Written Blocks": 0
        }
      ]
    },
    "Planning": {
      "Shared Hit Blocks": 101,
      "Shared Read Blocks": 5,
      "Shared Dirtied Blocks": 2,
      "Shared Written Blocks": 0,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 0,
      "Temp Written Blocks": 0
    },
    "Planning Time": 0.902,
    "Triggers": [
    ],
    "Execution Time": 76644.337
  }
]

//now disabling indexes

test=# UPDATE pg_index set indisready=false WHERE indrelid = ( SELECT oid FROM pg_class WHERE relname='t_acte_entite' );                     
UPDATE 9

//first run with index disabled

test=# explain (analyse, COSTS, buffers, verbose, FORMAT JSON) update t_acte_entite set acte_prevu = true where (fk_ssiad_visite_planning is not null or fk_ssiad_visite is not null);
[
  {
    "Plan": {
      "Node Type": "ModifyTable",
      "Operation": "Update",
      "Parallel Aware": false,
      "Relation Name": "t_acte_entite",
      "Schema": "public",
      "Alias": "t_acte_entite",
      "Startup Cost": 0.00,
      "Total Cost": 75577.00,
      "Plan Rows": 2794251,
      "Plan Width": 46,
      "Actual Startup Time": 12513.195,
      "Actual Total Time": 12513.195,
      "Actual Rows": 0,
      "Actual Loops": 1,
      "Shared Hit Blocks": 9228453,
      "Shared Read Blocks": 54260,
      "Shared Dirtied Blocks": 44853,
      "Shared Written Blocks": 27665,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 0,
      "Temp Written Blocks": 0,
      "Plans": [
        {
          "Node Type": "Seq Scan",
          "Parent Relationship": "Member",
          "Parallel Aware": false,
          "Relation Name": "t_acte_entite",
          "Schema": "public",
          "Alias": "t_acte_entite",
          "Startup Cost": 0.00,
          "Total Cost": 75577.00,
          "Plan Rows": 2794251,
          "Plan Width": 46,
          "Actual Startup Time": 0.049,
          "Actual Total Time": 807.352,
          "Actual Rows": 3049406,
          "Actual Loops": 1,
          "Output": ["id_acte_entite", "fk_acte", "fk_rubrique", "fk_client", "fk_salarie", "fk_cycldet", "fk_planning", "act_user_modified", "fk_ssiad_visite", "fk_ssiad_visite_planning", "true", "acte_realise", "acte_realise_modifie", "ctid"],
          "Filter": "((t_acte_entite.fk_ssiad_visite_planning IS NOT NULL) OR (t_acte_entite.fk_ssiad_visite IS NOT NULL))",
          "Rows Removed by Filter": 477,
          "Shared Hit Blocks": 13018,
          "Shared Read Blocks": 31830,
          "Shared Dirtied Blocks": 0,
          "Shared Written Blocks": 13868,
          "Local Hit Blocks": 0,
          "Local Read Blocks": 0,
          "Local Dirtied Blocks": 0,
          "Local Written Blocks": 0,
          "Temp Read Blocks": 0,
          "Temp Written Blocks": 0
        }
      ]
    },
    "Planning": {
      "Shared Hit Blocks": 147,
      "Shared Read Blocks": 6,
      "Shared Dirtied Blocks": 0,
      "Shared Written Blocks": 0,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 0,
      "Temp Written Blocks": 0
    },
    "Planning Time": 1.286,
    "Triggers": [
    ],
    "Execution Time": 12516.468
  }
]

//second run

test=# explain (analyse, COSTS, buffers, verbose, FORMAT JSON) update t_acte_entite set acte_prevu = true where (fk_ssiad_visite_planning is not null or fk_ssiad_visite is not null);
[
  {
    "Plan": {
      "Node Type": "ModifyTable",
      "Operation": "Update",
      "Parallel Aware": false,
      "Relation Name": "t_acte_entite",
      "Schema": "public",
      "Alias": "t_acte_entite",
      "Startup Cost": 0.00,
      "Total Cost": 75577.00,
      "Plan Rows": 2794251,
      "Plan Width": 46,
      "Actual Startup Time": 18273.687,
      "Actual Total Time": 18273.688,
      "Actual Rows": 0,
      "Actual Loops": 1,
      "Shared Hit Blocks": 9214814,
      "Shared Read Blocks": 43530,
      "Shared Dirtied Blocks": 51243,
      "Shared Written Blocks": 55114,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 0,
      "Temp Written Blocks": 0,
      "Plans": [
        {
          "Node Type": "Seq Scan",
          "Parent Relationship": "Member",
          "Parallel Aware": false,
          "Relation Name": "t_acte_entite",
          "Schema": "public",
          "Alias": "t_acte_entite",
          "Startup Cost": 0.00,
          "Total Cost": 75577.00,
          "Plan Rows": 2794251,
          "Plan Width": 46,
          "Actual Startup Time": 50.822,
          "Actual Total Time": 959.917,
          "Actual Rows": 3049406,
          "Actual Loops": 1,
          "Output": ["id_acte_entite", "fk_acte", "fk_rubrique", "fk_client", "fk_salarie", "fk_cycldet", "fk_planning", "act_user_modified", "fk_ssiad_visite", "fk_ssiad_visite_planning", "true", "acte_realise", "acte_realise_modifie", "ctid"],
          "Filter": "((t_acte_entite.fk_ssiad_visite_planning IS NOT NULL) OR (t_acte_entite.fk_ssiad_visite IS NOT NULL))",
          "Rows Removed by Filter": 477,
          "Shared Hit Blocks": 1324,
          "Shared Read Blocks": 43524,
          "Shared Dirtied Blocks": 9507,
          "Shared Written Blocks": 19397,
          "Local Hit Blocks": 0,
          "Local Read Blocks": 0,
          "Local Dirtied Blocks": 0,
          "Local Written Blocks": 0,
          "Temp Read Blocks": 0,
          "Temp Written Blocks": 0
        }
      ]
    },
    "Planning": {
      "Shared Hit Blocks": 0,
      "Shared Read Blocks": 0,
      "Shared Dirtied Blocks": 0,
      "Shared Written Blocks": 0,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 0,
      "Temp Written Blocks": 0
    },
    "Planning Time": 0.192,
    "Triggers": [
    ],
    "Execution Time": 18276.726
  }
]
test=#

If no one has an idea I will try to make a simple reproducer with a ligth schema

Thanks

Philippe