set @tmp_opt_switch= @@optimizer_switch;
set optimizer_switch='index_merge_sort_intersection=on';
set optimizer_trace='enabled=on';
create table t0 (a int);
insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
create table t1 (a int, b int, c int, filler char(100),
key(a), key(b), key(c)) charset=latin1;
insert into t1 select
A.a * B.a*10 + C.a*100,
A.a * B.a*10 + C.a*100,
A.a,
'filler'
from t0 A, t0 B, t0 C;
This should use union:
explain select * from t1 where a=1 or b=1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index_merge a,b a,b 5,5 NULL 2 Using union(a,b); Using where
select * from information_schema.OPTIMIZER_TRACE;
QUERY TRACE MISSING_BYTES_BEYOND_MAX_MEM_SIZE INSUFFICIENT_PRIVILEGES
explain select * from t1 where a=1 or b=1 {
  "steps": [
    {
      "join_preparation": {
        "select_id": 1,
        "steps": [
          {
            "expanded_query": "select t1.a AS a,t1.b AS b,t1.c AS c,t1.filler AS filler from t1 where t1.a = 1 or t1.b = 1"
          }
        ]
      }
    },
    {
      "join_optimization": {
        "select_id": 1,
        "steps": [
          {
            "condition_processing": {
              "condition": "WHERE",
              "original_condition": "t1.a = 1 or t1.b = 1",
              "steps": [
                {
                  "transformation": "equality_propagation",
                  "resulting_condition": "multiple equal(1, t1.a) or multiple equal(1, t1.b)"
                },
                {
                  "transformation": "constant_propagation",
                  "resulting_condition": "multiple equal(1, t1.a) or multiple equal(1, t1.b)"
                },
                {
                  "transformation": "trivial_condition_removal",
                  "resulting_condition": "multiple equal(1, t1.a) or multiple equal(1, t1.b)"
                }
              ]
            }
          },
          {
            "ref_optimizer_key_uses": []
          },
          {
            "rows_estimation": [
              {
                "table": "t1",
                "range_analysis": {
                  "table_scan": {
                    "rows": 1000,
                    "cost": 0.1729314
                  },
                  "potential_range_indexes": [
                    {
                      "index": "a",
                      "usable": true,
                      "key_parts": ["a"]
                    },
                    {
                      "index": "b",
                      "usable": true,
                      "key_parts": ["b"]
                    },
                    {
                      "index": "c",
                      "usable": false,
                      "cause": "not applicable"
                    }
                  ],
                  "setup_range_conditions": [],
                  "analyzing_range_alternatives": {
                    "range_scan_alternatives": [],
                    "analyzing_roworder_intersect": {
                      "cause": "too few roworder scans"
                    },
                    "analyzing_sort_intersect": {
                      "cutoff_cost": 0.1729314
                    },
                    "analyzing_index_merge_union": [
                      {
                        "indexes_to_merge": [
                          {
                            "range_scan_alternatives": [
                              {
                                "index": "a",
                                "ranges": ["(1) <= (a) <= (1)"],
                                "rowid_ordered": true,
                                "using_mrr": false,
                                "index_only": true,
                                "rows": 1,
                                "cost": 0.001478954,
                                "chosen": true
                              }
                            ],
                            "index_to_merge": "a",
                            "cumulated_cost": 0.001478954
                          },
                          {
                            "range_scan_alternatives": [
                              {
                                "index": "b",
                                "ranges": ["(1) <= (b) <= (1)"],
                                "rowid_ordered": true,
                                "using_mrr": false,
                                "index_only": true,
                                "rows": 1,
                                "cost": 0.001478954,
                                "chosen": true
                              }
                            ],
                            "index_to_merge": "b",
                            "cumulated_cost": 0.002957908
                          }
                        ],
                        "cost_of_reading_ranges": 0.002957908,
                        "use_roworder_union": true,
                        "cause": "always cheaper than non roworder retrieval",
                        "analyzing_roworder_scans": [
                          {
                            "type": "range_scan",
                            "index": "a",
                            "rows": 1,
                            "ranges": ["(1) <= (a) <= (1)"],
                            "analyzing_roworder_intersect": {
                              "cause": "too few roworder scans"
                            }
                          },
                          {
                            "type": "range_scan",
                            "index": "b",
                            "rows": 1,
                            "ranges": ["(1) <= (b) <= (1)"],
                            "analyzing_roworder_intersect": {
                              "cause": "too few roworder scans"
                            }
                          }
                        ],
                        "index_roworder_union_cost": 0.005185782,
                        "members": 2,
                        "chosen": true
                      }
                    ]
                  },
                  "group_index_range": {
                    "chosen": false,
                    "cause": "no group by or distinct"
                  },
                  "chosen_range_access_summary": {
                    "range_access_plan": {
                      "type": "index_roworder_union",
                      "union_of": [
                        {
                          "type": "range_scan",
                          "index": "a",
                          "rows": 1,
                          "ranges": ["(1) <= (a) <= (1)"]
                        },
                        {
                          "type": "range_scan",
                          "index": "b",
                          "rows": 1,
                          "ranges": ["(1) <= (b) <= (1)"]
                        }
                      ]
                    },
                    "rows_for_plan": 2,
                    "cost_for_plan": 0.005185782,
                    "chosen": true
                  }
                }
              },
              {
                "selectivity_for_indexes": [
                  {
                    "use_opt_range_condition_rows_selectivity": 0.002
                  }
                ],
                "selectivity_for_columns": [],
                "cond_selectivity": 0.002
              }
            ]
          },
          {
            "table_dependencies": [
              {
                "table": "t1",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": []
              }
            ]
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": "",
                "get_costs_for_tables": [
                  {
                    "best_access_path": {
                      "table": "t1",
                      "plan_details": {
                        "record_count": 1
                      },
                      "considered_access_paths": [
                        {
                          "access_type": "index_merge",
                          "rows": 2,
                          "rows_after_filter": 2,
                          "rows_out": 2,
                          "cost": 0.005185782,
                          "chosen": true
                        }
                      ],
                      "chosen_access_method": {
                        "type": "index_merge",
                        "rows_read": 2,
                        "rows_out": 2,
                        "cost": 0.005185782,
                        "uses_join_buffering": false
                      }
                    }
                  }
                ]
              },
              {
                "plan_prefix": "",
                "table": "t1",
                "rows_for_plan": 2,
                "cost_for_plan": 0.005185782
              }
            ]
          },
          {
            "best_join_order": ["t1"],
            "rows": 2,
            "cost": 0.005185782
          },
          {
            "substitute_best_equal": {
              "condition": "WHERE",
              "resulting_condition": "t1.a = 1 or t1.b = 1"
            }
          },
          {
            "attaching_conditions_to_tables": {
              "attached_conditions_computation": [],
              "attached_conditions_summary": [
                {
                  "table": "t1",
                  "attached_condition": "t1.a = 1 or t1.b = 1"
                }
              ]
            }
          },
          {
            "make_join_readinfo": []
          }
        ]
      }
    }
  ]
} 0 0
drop table t0,t1;
set optimizer_trace="enabled=off";
set @@optimizer_switch= @tmp_opt_switch;
# More tests added index_merge access
create table t1
(
/* Field names reflect value(rowid) distribution, st=STairs, swt= SaWTooth */
st_a int not null default 0,
swt1a int not null default 0,
swt2a int not null default 0,
st_b int not null default 0,
swt1b int not null default 0,
swt2b int not null default 0,
/* fields/keys for row retrieval tests */
key1 int,
key2 int,
key3 int,
key4 int,
/* make rows much bigger then keys */
filler1 char (200),
filler2 char (200),
filler3 char (200),
filler4 char (200),
filler5 char (200),
filler6 char (200),
/* order of keys is important */
key sta_swt12a(st_a,swt1a,swt2a),
key sta_swt1a(st_a,swt1a),
key sta_swt2a(st_a,swt2a),
key sta_swt21a(st_a,swt2a,swt1a),
key st_a(st_a),
key stb_swt1a_2b(st_b,swt1b,swt2a),
key stb_swt1b(st_b,swt1b),
key st_b(st_b),
key(key1),
key(key2),
key(key3),
key(key4)
) ;
create table t0 as select * from t1;
# Printing of many insert into t0 values (....) disabled.
alter table t1 disable keys;
# Printing of many insert into t1 select .... from t0 disabled.
# Printing of many insert into t1 (...) values (....) disabled.
alter table t1 enable keys;
insert into t1 (key1, key2, key3, key4, filler1) values (100, 100, -1, -1, 'key1-key2');
insert into t1 (key1, key2, key3, key4, filler1) values (-1, -1, 100, 100, 'key4-key3');
set optimizer_trace='enabled=on';
#  3-way ROR-intersection
explain select key1,key2,key3 from t1 where key1=100 and key2=100 and key3=100;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index_merge key1,key2,key3 key1,key2 5,5 NULL 77 Using intersect(key1,key2); Using where
select JSON_DETAILED(JSON_EXTRACT(trace, '$**.analyzing_range_alternatives')) AS JS from INFORMATION_SCHEMA.OPTIMIZER_TRACE;
JS
[
    {
        "range_scan_alternatives": 
        [
            {
                "index": "key1",
                "ranges": 
                ["(100) <= (key1) <= (100)"],
                "rowid_ordered": true,
                "using_mrr": false,
                "index_only": false,
                "rows": 2243,
                "cost": 2.770351251,
                "chosen": true
            },
            {
                "index": "key2",
                "ranges": 
                ["(100) <= (key2) <= (100)"],
                "rowid_ordered": true,
                "using_mrr": false,
                "index_only": false,
                "rows": 2243,
                "cost": 2.770351251,
                "chosen": false,
                "cause": "cost"
            },
            {
                "index": "key3",
                "ranges": 
                ["(100) <= (key3) <= (100)"],
                "rowid_ordered": true,
                "using_mrr": false,
                "index_only": false,
                "rows": 2243,
                "cost": 2.770351251,
                "chosen": false,
                "cause": "cost"
            }
        ],
        "analyzing_roworder_intersect": 
        {
            "intersecting_indexes": 
            [
                {
                    "index": "key1",
                    "index_scan_cost": 0.240986767,
                    "cumulated_index_scan_cost": 0.240986767,
                    "disk_sweep_cost": 2.564386012,
                    "cumulative_total_cost": 2.805372779,
                    "usable": true,
                    "matching_rows_now": 2243,
                    "intersect_covering_with_this_index": false,
                    "chosen": true
                },
                {
                    "index": "key2",
                    "index_scan_cost": 0.240986767,
                    "cumulated_index_scan_cost": 0.481973534,
                    "disk_sweep_cost": 0.089164506,
                    "cumulative_total_cost": 0.57113804,
                    "usable": true,
                    "matching_rows_now": 77.6360508,
                    "intersect_covering_with_this_index": false,
                    "chosen": true
                },
                {
                    "index": "key3",
                    "index_scan_cost": 0.240986767,
                    "cumulated_index_scan_cost": 0.722960301,
                    "disk_sweep_cost": 0,
                    "cumulative_total_cost": 0.722960301,
                    "usable": true,
                    "matching_rows_now": 2.687185191,
                    "intersect_covering_with_this_index": true,
                    "chosen": false,
                    "cause": "does not reduce cost"
                }
            ],
            "clustered_pk": 
            {
                "clustered_pk_added_to_intersect": false,
                "cause": "no clustered pk index"
            },
            "rows": 77,
            "cost": 0.573622393,
            "covering": false,
            "chosen": true
        }
    }
]
select JSON_DETAILED(JSON_EXTRACT(trace, '$**.chosen_range_access_summary')) AS JS from INFORMATION_SCHEMA.OPTIMIZER_TRACE;
JS
[
    {
        "range_access_plan": 
        {
            "type": "index_roworder_intersect",
            "rows": 77,
            "cost": 0.573622393,
            "covering": false,
            "clustered_pk_scan": false,
            "intersect_of": 
            [
                {
                    "type": "range_scan",
                    "index": "key1",
                    "rows": 2243,
                    "ranges": 
                    ["(100) <= (key1) <= (100)"]
                },
                {
                    "type": "range_scan",
                    "index": "key2",
                    "rows": 2243,
                    "ranges": 
                    ["(100) <= (key2) <= (100)"]
                }
            ]
        },
        "rows_for_plan": 77,
        "cost_for_plan": 0.573622393,
        "chosen": true
    }
]
# ROR-union(ROR-intersection, ROR-range)
explain select key1,key2,key3,key4 from t1 where key1=100 and key2=100 or key3=100 and key4=100;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index_merge key1,key2,key3,key4 key1,key2,key3,key4 5,5,5,5 NULL 154 Using union(intersect(key1,key2),intersect(key3,key4)); Using where
select JSON_DETAILED(JSON_EXTRACT(trace, '$**.analyzing_range_alternatives')) AS JS from INFORMATION_SCHEMA.OPTIMIZER_TRACE;
JS
[
    {
        "range_scan_alternatives": 
        [],
        "analyzing_roworder_intersect": 
        {
            "cause": "too few roworder scans"
        },
        "analyzing_index_merge_union": 
        [
            {
                "indexes_to_merge": 
                [
                    {
                        "range_scan_alternatives": 
                        [
                            {
                                "index": "key1",
                                "ranges": 
                                ["(100) <= (key1) <= (100)"],
                                "rowid_ordered": true,
                                "using_mrr": false,
                                "index_only": true,
                                "rows": 2243,
                                "cost": 0.312922694,
                                "chosen": true
                            },
                            {
                                "index": "key2",
                                "ranges": 
                                ["(100) <= (key2) <= (100)"],
                                "rowid_ordered": true,
                                "using_mrr": false,
                                "index_only": true,
                                "rows": 2243,
                                "cost": 0.312922694,
                                "chosen": false,
                                "cause": "cost"
                            }
                        ],
                        "index_to_merge": "key1",
                        "cumulated_cost": 0.312922694
                    },
                    {
                        "range_scan_alternatives": 
                        [
                            {
                                "index": "key3",
                                "ranges": 
                                ["(100) <= (key3) <= (100)"],
                                "rowid_ordered": true,
                                "using_mrr": false,
                                "index_only": true,
                                "rows": 2243,
                                "cost": 0.312922694,
                                "chosen": true
                            },
                            {
                                "index": "key4",
                                "ranges": 
                                ["(100) <= (key4) <= (100)"],
                                "rowid_ordered": true,
                                "using_mrr": false,
                                "index_only": true,
                                "rows": 2243,
                                "cost": 0.312922694,
                                "chosen": false,
                                "cause": "cost"
                            }
                        ],
                        "index_to_merge": "key3",
                        "cumulated_cost": 0.625845388
                    }
                ],
                "cost_of_reading_ranges": 0.625845388,
                "use_roworder_union": true,
                "cause": "always cheaper than non roworder retrieval",
                "analyzing_roworder_scans": 
                [
                    {
                        "type": "range_scan",
                        "index": "key1",
                        "rows": 2243,
                        "ranges": 
                        ["(100) <= (key1) <= (100)"],
                        "analyzing_roworder_intersect": 
                        {
                            "intersecting_indexes": 
                            [
                                {
                                    "index": "key1",
                                    "index_scan_cost": 0.240986767,
                                    "cumulated_index_scan_cost": 0.240986767,
                                    "disk_sweep_cost": 2.564386012,
                                    "cumulative_total_cost": 2.805372779,
                                    "usable": true,
                                    "matching_rows_now": 2243,
                                    "intersect_covering_with_this_index": false,
                                    "chosen": true
                                },
                                {
                                    "index": "key2",
                                    "index_scan_cost": 0.240986767,
                                    "cumulated_index_scan_cost": 0.481973534,
                                    "disk_sweep_cost": 0.089164506,
                                    "cumulative_total_cost": 0.57113804,
                                    "usable": true,
                                    "matching_rows_now": 77.6360508,
                                    "intersect_covering_with_this_index": false,
                                    "chosen": true
                                }
                            ],
                            "clustered_pk": 
                            {
                                "clustered_pk_added_to_intersect": false,
                                "cause": "no clustered pk index"
                            },
                            "rows": 77,
                            "cost": 0.573622393,
                            "covering": false,
                            "chosen": true
                        }
                    },
                    {
                        "type": "range_scan",
                        "index": "key3",
                        "rows": 2243,
                        "ranges": 
                        ["(100) <= (key3) <= (100)"],
                        "analyzing_roworder_intersect": 
                        {
                            "intersecting_indexes": 
                            [
                                {
                                    "index": "key3",
                                    "index_scan_cost": 0.240986767,
                                    "cumulated_index_scan_cost": 0.240986767,
                                    "disk_sweep_cost": 2.564386012,
                                    "cumulative_total_cost": 2.805372779,
                                    "usable": true,
                                    "matching_rows_now": 2243,
                                    "intersect_covering_with_this_index": false,
                                    "chosen": true
                                },
                                {
                                    "index": "key4",
                                    "index_scan_cost": 0.240986767,
                                    "cumulated_index_scan_cost": 0.481973534,
                                    "disk_sweep_cost": 0.089164506,
                                    "cumulative_total_cost": 0.57113804,
                                    "usable": true,
                                    "matching_rows_now": 77.6360508,
                                    "intersect_covering_with_this_index": false,
                                    "chosen": true
                                }
                            ],
                            "clustered_pk": 
                            {
                                "clustered_pk_added_to_intersect": false,
                                "cause": "no clustered pk index"
                            },
                            "rows": 77,
                            "cost": 0.573622393,
                            "covering": false,
                            "chosen": true
                        }
                    }
                ],
                "index_roworder_union_cost": 1.135493366,
                "members": 2,
                "chosen": true
            }
        ]
    }
]
select JSON_DETAILED(JSON_EXTRACT(trace, '$**.chosen_range_access_summary')) AS JS from INFORMATION_SCHEMA.OPTIMIZER_TRACE;
JS
[
    {
        "range_access_plan": 
        {
            "type": "index_roworder_union",
            "union_of": 
            [
                {
                    "type": "index_roworder_intersect",
                    "rows": 77,
                    "cost": 0.573622393,
                    "covering": false,
                    "clustered_pk_scan": false,
                    "intersect_of": 
                    [
                        {
                            "type": "range_scan",
                            "index": "key1",
                            "rows": 2243,
                            "ranges": 
                            ["(100) <= (key1) <= (100)"]
                        },
                        {
                            "type": "range_scan",
                            "index": "key2",
                            "rows": 2243,
                            "ranges": 
                            ["(100) <= (key2) <= (100)"]
                        }
                    ]
                },
                {
                    "type": "index_roworder_intersect",
                    "rows": 77,
                    "cost": 0.573622393,
                    "covering": false,
                    "clustered_pk_scan": false,
                    "intersect_of": 
                    [
                        {
                            "type": "range_scan",
                            "index": "key3",
                            "rows": 2243,
                            "ranges": 
                            ["(100) <= (key3) <= (100)"]
                        },
                        {
                            "type": "range_scan",
                            "index": "key4",
                            "rows": 2243,
                            "ranges": 
                            ["(100) <= (key4) <= (100)"]
                        }
                    ]
                }
            ]
        },
        "rows_for_plan": 154,
        "cost_for_plan": 1.135493366,
        "chosen": true
    }
]
drop table t0,t1;
set optimizer_trace="enabled=off";