perf(gen): use JOIN DISTINCT unnest(...) for composite-key loaders on psql

Composite-key relationship loaders and LoadCount used `(a, b) IN (SELECT
unnest($1), unnest($2))`, which mis-estimates row counts and forces a Seq
Scan + Hash Semi Join. Switch to an INNER JOIN on a DISTINCT unnest so the
planner can use an indexed nested loop. Single-column keys are unchanged.

Author: sandonemaki

gen/templates/counts/table/115_counts.go.tpl

@@ -208,13 +208,6 @@ func (os {{$tAlias.UpSingular}}Slice) LoadCount{{$relAlias}}(ctx context.Context
 	{{- $fromCol := index $firstFrom.Columns $local}}
 	PKArgExpr := {{$.Dialect}}.Any({{$.Dialect}}.Cast({{$.Dialect}}.Arg(pk{{$fromCol}}), "{{$column.DBType}}[]"))
 	{{- else}}
-	PKArgExpr := {{$.Dialect}}.Select(sm.Columns(
-		{{- range $index, $local := $firstSide.FromColumns -}}
-		{{- $column := $.Table.GetColumn $local -}}
-		{{- $fromCol := index $firstFrom.Columns $local}}
-		{{$.Dialect}}.F("unnest", {{$.Dialect}}.Cast({{$.Dialect}}.Arg(pk{{$fromCol}}), "{{$column.DBType}}[]")),
-		{{- end}}
-	))
 	{{- end}}
 	{{- end}}
 
@@ -286,6 +279,36 @@ func (os {{$tAlias.UpSingular}}Slice) LoadCount{{$relAlias}}(ctx context.Context
 		{{- else -}}
 		sm.Where({{$firstTo.UpPlural}}.Columns.{{$firstToColAlias}}.OP("IN", PKArgExpr)),
 		{{- end}}
+		{{- else if eq $.Dialect "psql" -}}
+		sm.InnerJoin({{$.Dialect}}.Select(
+			sm.Distinct(),
+			sm.Columns(
+				{{range $index, $local := $firstSide.FromColumns -}}
+				{{$toLocal := index $firstSide.ToColumns $index -}}
+				{{$firstToColAlias := index $firstTo.Columns $toLocal -}}
+				{{$.Dialect}}.Quote("bob_rel_keys_src", {{quote $firstToColAlias}}),
+				{{end -}}
+			),
+			sm.From({{$.Dialect}}.F("unnest",
+				{{range $index, $local := $firstSide.FromColumns -}}
+				{{$column := $.Table.GetColumn $local -}}
+				{{$fromCol := index $firstFrom.Columns $local -}}
+				{{$.Dialect}}.Cast({{$.Dialect}}.Arg(pk{{$fromCol}}), "{{$column.DBType}}[]"),
+				{{end -}}
+			)).As("bob_rel_keys_src"
+				{{- range $index, $local := $firstSide.FromColumns -}}
+				{{- $toLocal := index $firstSide.ToColumns $index -}}
+				{{- $firstToColAlias := index $firstTo.Columns $toLocal -}}
+				, {{quote $firstToColAlias}}
+				{{- end -}}
+			),
+		)).As("bob_rel_keys").On(
+			{{range $index, $local := $firstSide.FromColumns -}}
+			{{$toLocal := index $firstSide.ToColumns $index -}}
+			{{$firstToColAlias := index $firstTo.Columns $toLocal -}}
+			{{$firstTo.UpPlural}}.Columns.{{$firstToColAlias}}.EQ({{$.Dialect}}.Quote("bob_rel_keys", {{quote $firstToColAlias}})),
+			{{end -}}
+		),
 		{{- else -}}
 		sm.Where({{$.Dialect}}.Group(
 			{{range $index, $local := $firstSide.FromColumns -}}

gen/templates/models/table/009_rel_query.go.tpl

@@ -93,13 +93,6 @@ func (os {{$tAlias.UpSingular}}Slice) {{relQueryMethodName $tAlias $relAlias}}(m
     {{- $fromCol := index $firstFrom.Columns $local}}
     PKArgExpr := psql.Any(psql.Cast(psql.Arg(pk{{$fromCol}}), "{{$column.DBType}}[]"))
     {{else}}
-    PKArgExpr := psql.Select(sm.Columns(
-      {{- range $index, $local := $firstSide.FromColumns -}}
-        {{$column := $.Table.GetColumn $local}}
-        {{$fromCol := index $firstFrom.Columns $local -}}
-        psql.F("unnest", psql.Cast(psql.Arg(pk{{$fromCol}}), "{{$column.DBType}}[]")),
-      {{- end}}
-    ))
     {{end}}
   {{end}}
 	{{- end}}
@@ -135,6 +128,33 @@ func (os {{$tAlias.UpSingular}}Slice) {{relQueryMethodName $tAlias $relAlias}}(m
 				{{if and (eq $.Dialect "psql") (eq (len $side.FromColumns) 1) -}}
 					{{- $toCol := index $to.Columns (index $side.ToColumns 0) -}}
 					sm.Where({{$to.UpPlural}}.Columns.{{$toCol}}.EQ(PKArgExpr)),
+				{{- else if eq $.Dialect "psql" -}}
+					sm.InnerJoin(psql.Select(
+						sm.Distinct(),
+						sm.Columns(
+							{{- range $index, $local := $side.FromColumns -}}
+							{{- $toCol := index $to.Columns (index $side.ToColumns $index) -}}
+							psql.Quote("bob_rel_keys_src", {{quote $toCol}}),
+							{{- end}}
+						),
+						sm.From(psql.F("unnest",
+							{{- range $index, $local := $side.FromColumns -}}
+							{{- $fromCol := index $from.Columns $local -}}
+							{{- $column := $.Table.GetColumn $local -}}
+							psql.Cast(psql.Arg(pk{{$fromCol}}), "{{$column.DBType}}[]"),
+							{{- end}}
+						)).As("bob_rel_keys_src"
+							{{- range $index, $local := $side.FromColumns -}}
+							{{- $toCol := index $to.Columns (index $side.ToColumns $index) -}}
+							, {{quote $toCol}}
+							{{- end -}}
+						),
+					)).As("bob_rel_keys").On(
+						{{- range $index, $local := $side.FromColumns -}}
+						{{- $toCol := index $to.Columns (index $side.ToColumns $index) -}}
+						{{$to.UpPlural}}.Columns.{{$toCol}}.EQ(psql.Quote("bob_rel_keys", {{quote $toCol}})),
+						{{- end}}
+					),
 				{{- else -}}
 					sm.Where({{$.Dialect}}.Group(
 					{{- range $index, $local := $side.FromColumns -}}