You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 20230516154000_drop_reference_sha.go
    path of file: ./repos/dendrite/roomserver/storage/postgres/deltas
    
    the code of the file until where you have to start completion: // Copyright 2023 The Matrix.org Foundation C.I.C.
//
// 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 deltas

import (
	"context"
	"database/sql"
	"fmt"

	"github.com/lib/pq"
	"github.com/matrix-org/dendrite/internal"
	"github.com/matrix-org/util"
)

func UpDropEventReferenceSHAEvents(ctx context.Context, tx *sql.Tx) error {
	_, err := tx.ExecContext(ctx, `ALTER TABLE roomserver_events DROP COLUMN IF EXISTS reference_sha256;`)
	if err != nil {
		return fmt.Errorf("failed to execute upgrade: %w", err)
	}
	return nil
}

func UpDropEventReferenceSHAPrevEvents(ctx context.Context, tx *sql.Tx) error {
	_, err := tx.ExecContext(ctx, "ALTER TABLE roomserver_previous_events DROP CONSTRAINT roomserver_previous_event_id_unique;")
	if err != nil {
		return fmt.Errorf("failed to execute upgrade: %w", err)
	}
	_, err = tx.ExecContext(ctx, `ALTER TABLE roomserver_previous_events DROP COLUMN IF EXISTS previous_reference_sha256;`)
	if err != nil {
		return fmt.Errorf("failed to execute upgrade: %w", err)
	}

	// figure out if there are duplicates
	dupeRows, err := tx.QueryContext(ctx, `SELECT previous_event_id FROM roomserver_previous_events GROUP BY previous_event_id HAVING count(previous_event_id) > 1`)
	if err != nil {
		return fmt.Errorf("failed to query duplicate eve
    
func UpDropEventReferenceSHAPrevEvents(ctx context.Context, tx *sql.Tx) error {
	_, err := tx.ExecContext(ctx, "ALTER TABLE roomserver_previous_events DROP CONSTRAINT roomserver_previous_event_id_unique;")
	if err != nil {
		return fmt.Errorf("failed to execute upgrade: %w", err)
	}
	_, err = tx.ExecContext(ctx, `ALTER TABLE roomserver_previous_events DROP COLUMN IF EXISTS previous_reference_sha256;`)
	if err != nil {
		return fmt.Errorf("failed to execute upgrade: %w", err)
	}

	// figure out if there are duplicates
	dupeRows, err := tx.QueryContext(ctx, `SELECT previous_event_id FROM roomserver_previous_events GROUP BY previous_event_id HAVING count(previous_event_id) > 1`)
	if err != nil {
		return fmt.Errorf("failed to query duplicate event ids")
	}
	defer internal.CloseAndLogIfError(ctx, dupeRows, "failed to close rows")

	var prevEvents []string
	var prevEventID string
	for dupeRows.Next() {
		if err = dupeRows.Scan(&prevEventID); err != nil {
			return err
		}
		prevEvents = append(prevEvents, prevEventID)
	}
	if dupeRows.Err() != nil {
		return dupeRows.Err()
	}

	// if we found duplicates, check if we can combine them, e.g. they are in the same room
	for _, dupeID := range prevEvents {
		var dupeNIDsRows *sql.Rows
		dupeNIDsRows, err = tx.QueryContext(ctx, `SELECT event_nids FROM roomserver_previous_events WHERE previous_event_id = $1`, dupeID)
		if err != nil {
			return fmt.Errorf("failed to query duplicate event ids")
		}
		defer internal.CloseAndLogIfError(ctx, dupeNIDsRows, "failed to close rows")
		var dupeNIDs []int64
		for dupeNIDsRows.Next() {
			var nids pq.Int64Array
			if err = dupeNIDsRows.Scan(&nids); err != nil {
				return err
			}
			dupeNIDs = append(dupeNIDs, nids...)
		}

		if dupeNIDsRows.Err() != nil {
			return dupeNIDsRows.Err()
		}
		// dedupe NIDs
		dupeNIDs = dupeNIDs[:util.SortAndUnique(nids(dupeNIDs))]
		// now that we have all NIDs, check which room they belong to
		var roomCount int
		err = tx.QueryRowContext(ctx, `SELECT count(distinct room_nid) FROM roomserver_events WHERE event_nid = ANY($1)`, pq.Array(dupeNIDs)).Scan(&roomCount)
		if err != nil {
			return err
		}
		// if the events are from different rooms, that's bad and we can't continue
		if roomCount > 1 {
			return fmt.Errorf("detected events (%v) referenced for different rooms (%v)", dupeNIDs, roomCount)
		}
		// otherwise delete the dupes
		_, err = tx.ExecContext(ctx, "DELETE FROM roomserver_previous_events WHERE previous_event_id = $1", dupeID)
		if err != nil {
			return fmt.Errorf("unable to delete duplicates: %w", err)
		}

		// insert combined values
		_, err = tx.ExecContext(ctx, "INSERT INTO roomserver_previous_events (previous_event_id, event_nids) VALUES ($1, $2)", dupeID, pq.Array(dupeNIDs))
		if err != nil {
			return fmt.Errorf("unable to insert new event NIDs: %w", err)
		}
	}

	_, err = tx.ExecContext(ctx, `ALTER TABLE roomserver_previous_events ADD CONSTRAINT roomserver_previous_event_id_unique UNIQUE (previous_event_id);`)
	if err != nil {
		return fmt.Errorf("failed to execute upgrade: %w", err)
	}
	return nil
}