Wrong fields being used on index

[mateusvmv]

Cozo is using the wrong fields with indexes in a specific scenario.

To reproduce, use the following schema: :create source { x => b } :create target { a => c, b } ::index create target:b { b, a }

And check the query plan

::explain {
    ?[a,b,c] :=
        *source{x,b},
        *target{a,b,c}
}

stratum	rule_idx	rule	atom_idx	op	ref	joins_on	filters/expr	out_relation
0	0	?	6	load_stored	:source	null	[]	["x","b"]
0	0	?	5	load_stored	:target:b	null	[]	["1","2"]
0	0	?	4	stored_prefix_join	null	{"b":"**1"}	null	["b","**2"]
0	0	?	3	load_stored	:target	null	[]	["a","c","**0"]
0	0	?	2	stored_mat_join	null	{"2":"0"}	null	["b","a","c"]
0	0	?	1	reorder	null	null	null	["a","b","c"]
0	0	?	0	out	null	null	null	["a","b","c"]

target:b has the format [b,a], so 2 is column a target has format [a, c, b], so 0 is column b The plan here is trying to join on two different columns

If we change the query by adding the index explicitly, it uses a correct plan, but includes b on the join. There is no need to include b, because a determines b. If b isn't right after a, it results in a matrix join, making the operation more expensive.

::explain {
    ?[a,b,c] :=
        *source{x,b},
        *target:b{a,b},
        *target{a,b,c}
}

stratum	rule_idx	rule	atom_idx	op	ref	joins_on	filters/expr	out_relation
0	0	?	6	load_stored	:source	null	[]	["x","b"]
0	0	?	5	load_stored	:target:b	null	[]	["**0","a"]
0	0	?	4	stored_prefix_join	null	{"b":"**0"}	null	["b","a"]
0	0	?	3	load_stored	:target	null	[]	["1","c","2"]
0	0	?	2	stored_mat_join	null	{"a":"1","b":"2"}	null	["b","a","c"]
0	0	?	1	reorder	null	null	null	["a","b","c"]
0	0	?	0	out	null	null	null	["a","b","c"]

The optimal plan is obtained with the following query

::explain {
    ?[a,b,c] :=
        *source{x,b},
        *target:b{a,b},
        *target{a,c}
}

stratum	rule_idx	rule	atom_idx	op	ref	joins_on	filters/expr	out_relation
0	0	?	6	load_stored	:source	null	[]	["x","b"]
0	0	?	5	load_stored	:target:b	null	[]	["**0","a"]
0	0	?	4	stored_prefix_join	null	{"b":"**0"}	null	["b","a"]
0	0	?	3	load_stored	:target	null	[]	["**1","c","~1"]
0	0	?	2	stored_prefix_join	null	{"a":"**1"}	null	["b","a","c"]
0	0	?	1	reorder	null	null	null	["a","b","c"]
0	0	?	0	out	null	null	null	["a","b","c"]

[mateusvmv]

This doubles up with another issue:

:create rel {x=>a,b}
::index create rel:a {a,x}
::explain { ?[a1,a2,a3] := *rel{a:a1,b:a2},*rel{a:a2,b:a3},*rel{a:a3,b:a1} }

This is supposed to bind two variables in the last query, but it only binds one. In that case, I think it should choose a prefix join on the index and a mat join on the relation, using three atoms, if that is acceptable.

I'm working on a fix for both and will come up with a PR soon.