From 2ddd7ada9b283d501337dced20aea3faa23f0018 Mon Sep 17 00:00:00 2001
From: Harvey Kandola <harvey@documize.com>
Date: Thu, 4 Apr 2019 12:08:57 +0100
Subject: [PATCH] Make storage providers generate row limit clause

---
 core/env/provider.go          |  3 +++
 domain/store/context.go       | 18 ++++++++++++++++++
 edition/storage/mysql.go      |  5 +++++
 edition/storage/postgresql.go |  5 +++++
 edition/storage/sqlserver.go  |  5 +++++
 5 files changed, 36 insertions(+)

diff --git a/core/env/provider.go b/core/env/provider.go
index 559aedd7..da314273 100644
--- a/core/env/provider.go
+++ b/core/env/provider.go
@@ -111,4 +111,7 @@ type StoreProvider interface {
 	// IsFalse returns storage provider boolean FALSE:
 	// MySQL is 0, PostgresSQL is FALSE, SQL Server is 0
 	IsFalse() string
+
+	// RowLimit returns SQL for limited number of returned rows
+	RowLimit(max int) string
 }
diff --git a/domain/store/context.go b/domain/store/context.go
index 383662a3..4f919a6e 100644
--- a/domain/store/context.go
+++ b/domain/store/context.go
@@ -119,3 +119,21 @@ func (c *Context) IsTrue() string {
 func (c *Context) IsFalse() string {
 	return c.Runtime.StoreProvider.IsFalse()
 }
+
+// RowLimitVariants returns the SQL clause for limiting rows.
+// Depending on the storage provider, the limit clause goes either
+// at the start of the query or the end.
+func (c *Context) RowLimitVariants(max int) (variantStart string, variantEnd string) {
+	variantStart = ""
+	variantEnd = ""
+
+	if c.Runtime.StoreProvider.Type() == env.StoreTypeSQLServer {
+		// SQL Server and variants use 'SELECT TOP nnn' syntax.
+		variantStart = c.Runtime.StoreProvider.RowLimit(max)
+	} else {
+		// MySQL and variants use 'SELECT ... LIMIT nnn' syntax.
+		variantEnd = c.Runtime.StoreProvider.RowLimit(max)
+	}
+
+	return
+}
diff --git a/edition/storage/mysql.go b/edition/storage/mysql.go
index 28b87c68..a9af2ec2 100644
--- a/edition/storage/mysql.go
+++ b/edition/storage/mysql.go
@@ -364,6 +364,11 @@ func (p MySQLProvider) IsFalse() string {
 	return "0"
 }
 
+// RowLimit returns SQL for limiting number of rows returned.
+func (p MySQLProvider) RowLimit(max int) string {
+	return fmt.Sprintf("LIMIT %d", max)
+}
+
 // convertDatabaseVersion turns database version string as major,minor,patch numerics.
 func convertDatabaseVersion(v string) (ints []int, err error) {
 	ints = []int{0, 0, 0}
diff --git a/edition/storage/postgresql.go b/edition/storage/postgresql.go
index effcc1cc..4695bd51 100644
--- a/edition/storage/postgresql.go
+++ b/edition/storage/postgresql.go
@@ -305,3 +305,8 @@ func (p PostgreSQLProvider) IsTrue() string {
 func (p PostgreSQLProvider) IsFalse() string {
 	return "false"
 }
+
+// RowLimit returns SQL for limiting number of rows returned.
+func (p PostgreSQLProvider) RowLimit(max int) string {
+	return fmt.Sprintf("LIMIT %d", max)
+}
diff --git a/edition/storage/sqlserver.go b/edition/storage/sqlserver.go
index 3baf323d..50ddfc1b 100644
--- a/edition/storage/sqlserver.go
+++ b/edition/storage/sqlserver.go
@@ -370,3 +370,8 @@ func (p SQLServerProvider) IsTrue() string {
 func (p SQLServerProvider) IsFalse() string {
 	return "0"
 }
+
+// RowLimit returns SQL for limiting number of rows returned.
+func (p SQLServerProvider) RowLimit(max int) string {
+	return fmt.Sprintf("TOP %d", max)
+}