From aabe5538a830c862ee07e2f2ea02731431403811 Mon Sep 17 00:00:00 2001
From: Jack Christensen <jack@jackchristensen.com>
Date: Sat, 9 May 2020 23:57:20 -0500
Subject: [PATCH] Optimize large result sets

---
 go.mod  |  2 +-
 go.sum  |  2 ++
 rows.go | 42 ++++++++++++++++++++++++++++++++++++++----
 3 files changed, 41 insertions(+), 5 deletions(-)

diff --git a/go.mod b/go.mod
index 17804c68..ef6e1568 100644
--- a/go.mod
+++ b/go.mod
@@ -9,7 +9,7 @@ require (
 	github.com/jackc/pgconn v1.5.0
 	github.com/jackc/pgio v1.0.0
 	github.com/jackc/pgproto3/v2 v2.0.1
-	github.com/jackc/pgtype v1.3.1-0.20200508211315-97bbe6ae20e2
+	github.com/jackc/pgtype v1.3.1-0.20200510045248-7e66ab1e146c
 	github.com/jackc/puddle v1.1.1
 	github.com/rs/zerolog v1.15.0
 	github.com/shopspring/decimal v0.0.0-20200227202807-02e2044944cc
diff --git a/go.sum b/go.sum
index 6cdc2fe3..84abfb55 100644
--- a/go.sum
+++ b/go.sum
@@ -57,6 +57,8 @@ github.com/jackc/pgtype v1.3.1-0.20200505182314-3b7c47a2a7da h1:ZbfsOjqJ1nHsryU0
 github.com/jackc/pgtype v1.3.1-0.20200505182314-3b7c47a2a7da/go.mod h1:vaogEUkALtxZMCH411K+tKzNpwzCKU+AnPzBKZ+I+Po=
 github.com/jackc/pgtype v1.3.1-0.20200508211315-97bbe6ae20e2 h1:Y6cErz3hUojOwnjUEWoZPRCBQcB7avM9ntGiYkB0wJo=
 github.com/jackc/pgtype v1.3.1-0.20200508211315-97bbe6ae20e2/go.mod h1:vaogEUkALtxZMCH411K+tKzNpwzCKU+AnPzBKZ+I+Po=
+github.com/jackc/pgtype v1.3.1-0.20200510045248-7e66ab1e146c h1:id5j6vOwHhbR7BYdGyb0sDMQjNsKTO+mXWaJxiwKu5M=
+github.com/jackc/pgtype v1.3.1-0.20200510045248-7e66ab1e146c/go.mod h1:vaogEUkALtxZMCH411K+tKzNpwzCKU+AnPzBKZ+I+Po=
 github.com/jackc/pgx/v4 v4.0.0-20190420224344-cc3461e65d96/go.mod h1:mdxmSJJuR08CZQyj1PVQBHy9XOp5p8/SHH6a0psbY9Y=
 github.com/jackc/pgx/v4 v4.0.0-20190421002000-1b8f0016e912/go.mod h1:no/Y67Jkk/9WuGR0JG/JseM9irFbnEPbuWV2EELPNuM=
 github.com/jackc/pgx/v4 v4.0.0-pre1.0.20190824185557-6972a5742186/go.mod h1:X+GQnOEnf1dqHGpw7JmHqHc1NxDoalibchSk9/RWuDc=
diff --git a/rows.go b/rows.go
index c16a64bd..91e231e3 100644
--- a/rows.go
+++ b/rows.go
@@ -109,6 +109,9 @@ type connRows struct {
 
 	resultReader      *pgconn.ResultReader
 	multiResultReader *pgconn.MultiResultReader
+
+	scanPlans []pgtype.ScanPlan
+	dstValues []interface{}
 }
 
 func (rows *connRows) FieldDescriptions() []pgproto3.FieldDescription {
@@ -184,11 +187,43 @@ func (rows *connRows) Next() bool {
 }
 
 func (rows *connRows) Scan(dest ...interface{}) error {
-	err := ScanRow(rows.connInfo, rows.FieldDescriptions(), rows.values, dest...)
-	if err != nil {
+	ci := rows.connInfo
+	fieldDescriptions := rows.FieldDescriptions()
+	values := rows.values
+
+	if len(fieldDescriptions) != len(values) {
+		err := errors.Errorf("number of field descriptions must equal number of values, got %d and %d", len(fieldDescriptions), len(values))
 		rows.fatal(err)
 		return err
 	}
+	if len(fieldDescriptions) != len(dest) {
+		err := errors.Errorf("number of field descriptions must equal number of destinations, got %d and %d", len(fieldDescriptions), len(dest))
+		rows.fatal(err)
+		return err
+	}
+
+	if rows.scanPlans == nil {
+		rows.scanPlans = make([]pgtype.ScanPlan, len(values))
+		rows.dstValues = make([]interface{}, len(values))
+	}
+
+	for i, dst := range dest {
+		if dst == nil {
+			continue
+		}
+
+		if dst != rows.dstValues[i] {
+			rows.scanPlans[i] = ci.PlanScan(fieldDescriptions[i].DataTypeOID, fieldDescriptions[i].Format, values[i], dest[i])
+			rows.dstValues[i] = dst
+		}
+
+		err := rows.scanPlans[i].Scan(ci, fieldDescriptions[i].DataTypeOID, fieldDescriptions[i].Format, values[i], dst)
+		if err != nil {
+			err = scanArgError{col: i, err: err}
+			rows.fatal(err)
+			return err
+		}
+	}
 
 	return nil
 }
@@ -278,8 +313,7 @@ func (e scanArgError) Error() string {
 	return fmt.Sprintf("can't scan into dest[%d]: %v", e.col, e.err)
 }
 
-// ScanRow decodes raw row data into dest. This is a low level function used internally to to implement the Rows
-// interface Scan method. It can be used to scan rows read from the lower level pgconn interface.
+// ScanRow decodes raw row data into dest. It can be used to scan rows read from the lower level pgconn interface.
 //
 // connInfo - OID to Go type mapping.
 // fieldDescriptions - OID and format of values