FerretDB · AlekSi · Oct 9, 2023 · Sep 29, 2023 · Sep 29, 2023 · Sep 29, 2023
@@ -28,6 +28,7 @@
 	"github.com/FerretDB/FerretDB/internal/backends/postgresql/metadata"
 	"github.com/FerretDB/FerretDB/internal/backends/postgresql/metadata/pool"
 	"github.com/FerretDB/FerretDB/internal/handlers/sjson"
+	"github.com/FerretDB/FerretDB/internal/types"
 	"github.com/FerretDB/FerretDB/internal/util/lazyerrors"
 	"github.com/FerretDB/FerretDB/internal/util/must"
 )
@@ -55,6 +56,10 @@
 		return nil, lazyerrors.Error(err)
 	}
 
+	if params == nil {
+		params = new(backends.QueryParams)
+	}
+
 	if p == nil {
 		return &backends.QueryResult{
 			Iter: newQueryIterator(ctx, nil),
@@ -72,14 +77,22 @@
 		}, nil
 	}
 
-	// TODO https://github.com/FerretDB/FerretDB/issues/3414
 	q := fmt.Sprintf(
 		`SELECT %s FROM %s`,
 		metadata.DefaultColumn,
 		pgx.Identifier{c.dbName, meta.TableName}.Sanitize(),
 	)
 
-	rows, err := p.Query(ctx, q)
+	var placeholder Placeholder
+
+	where, args, err := prepareWhereClause(&placeholder, params.Filter)
+	if err != nil {
+		return nil, lazyerrors.Error(err)
+	}
+
+	q += where
+
+	rows, err := p.Query(ctx, q, args...)
 	if err != nil {
 		return nil, lazyerrors.Error(err)
 	}
@@ -245,8 +258,59 @@
 
 // Explain implements backends.Collection interface.
 func (c *collection) Explain(ctx context.Context, params *backends.ExplainParams) (*backends.ExplainResult, error) {
-	// TODO https://github.com/FerretDB/FerretDB/issues/3389
-	return new(backends.ExplainResult), nil
+	p, err := c.r.DatabaseGetExisting(ctx, c.dbName)
+	if err != nil {
+		return nil, lazyerrors.Error(err)
+	}
+
+	res := new(backends.ExplainResult)
+
+	if p == nil {
+		return res, nil
+	}
+
+	meta, err := c.r.CollectionGet(ctx, c.dbName, c.name)
+	if err != nil {
+		return nil, lazyerrors.Error(err)
+	}
+
+	if meta == nil {
+		res.QueryPlanner = must.NotFail(types.NewDocument())
+		return res, nil
+	}
+
+	q := fmt.Sprintf(
+		`EXPLAIN (VERBOSE true, FORMAT JSON) SELECT %s FROM %s`,
+		metadata.DefaultColumn,
+		pgx.Identifier{c.dbName, meta.TableName}.Sanitize(),
+	)
+
+	var placeholder Placeholder
+
+	where, args, err := prepareWhereClause(&placeholder, params.Filter)
+	if err != nil {
+		return nil, lazyerrors.Error(err)
+	}
+
+	res.QueryPushdown = where != ""
+
+	q += where
+
+	var b []byte
+	err = p.QueryRow(ctx, q, args...).Scan(&b)
+
+	if err != nil {
+		return nil, lazyerrors.Error(err)
+	}
+
+	queryPlan, err := unmarshalExplain(b)
+	if err != nil {
+		return nil, lazyerrors.Error(err)
+	}
+
+	res.QueryPlanner = queryPlan
+
+	return res, nil
 }
 
 // Stats implements backends.Collection interface.

diff --git a/internal/backends/postgresql/helpers.go b/internal/backends/postgresql/helpers.go
@@ -0,0 +1,74 @@
+// Copyright 2021 FerretDB Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package postgresql
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+
+	"golang.org/x/exp/maps"
+
+	"github.com/FerretDB/FerretDB/internal/types"
+	"github.com/FerretDB/FerretDB/internal/util/lazyerrors"
+)
+
+// unmarshalExplain unmarshalls the plan from EXPLAIN postgreSQL command.
+// EXPLAIN result is not sjson, so it cannot be unmarshalled by sjson.Unmarshal.
+func unmarshalExplain(b []byte) (*types.Document, error) {
+	var plans []map[string]any
+	if err := json.Unmarshal(b, &plans); err != nil {
+		return nil, lazyerrors.Error(err)
+	}
+
+	if len(plans) == 0 {
+		return nil, lazyerrors.Error(errors.New("no execution plan returned"))
+	}
+
+	return convertJSON(plans[0]).(*types.Document), nil
+}
+
+// convertJSON transforms decoded JSON map[string]any value into *types.Document.
+func convertJSON(value any) any {
+	switch value := value.(type) {
+	case map[string]any:
+		d := types.MakeDocument(len(value))
+		keys := maps.Keys(value)
+
+		for _, k := range keys {
+			v := value[k]
+			d.Set(k, convertJSON(v))
+		}
+
+		return d
+
+	case []any:
+		a := types.MakeArray(len(value))
+		for _, v := range value {
+			a.Append(convertJSON(v))
+		}
+
+		return a
+
+	case nil:
+		return types.Null
+
+	case float64, string, bool:
+		return value
+
+	default:
+		panic(fmt.Sprintf("unsupported type: %[1]T (%[1]v)", value))
+	}
+}