diff --git a/src/dsql/ExprNodes.cpp b/src/dsql/ExprNodes.cpp
index bcac5b55f6..a6681398b5 100644
--- a/src/dsql/ExprNodes.cpp
+++ b/src/dsql/ExprNodes.cpp
@@ -63,6 +63,7 @@ using namespace Jrd;
 namespace Jrd {
 
 
+static const long LONG_POS_MAX = 2147483647;
 static const SINT64 MAX_INT64_LIMIT = MAX_SINT64 / 10;
 static const SINT64 MIN_INT64_LIMIT = MIN_SINT64 / 10;
 static const SINT64 SECONDS_PER_DAY = 24 * 60 * 60;
@@ -9492,9 +9493,20 @@ bool SubstringNode::setParameterType(DsqlCompilerScratch* dsqlScratch,
 void SubstringNode::genBlr(DsqlCompilerScratch* dsqlScratch)
 {
 	dsqlScratch->appendUChar(blr_substring);
+
 	GEN_expr(dsqlScratch, expr);
 	GEN_expr(dsqlScratch, start);
-	GEN_expr(dsqlScratch, length);
+
+	if (length)
+		GEN_expr(dsqlScratch, length);
+	else
+	{
+		dsqlScratch->appendUChar(blr_literal);
+		dsqlScratch->appendUChar(blr_long);
+		dsqlScratch->appendUChar(0);
+		dsqlScratch->appendUShort(LONG_POS_MAX);
+		dsqlScratch->appendUShort(LONG_POS_MAX >> 16);
+	}
 }
 
 void SubstringNode::make(DsqlCompilerScratch* dsqlScratch, dsc* desc)
@@ -9503,9 +9515,10 @@ void SubstringNode::make(DsqlCompilerScratch* dsqlScratch, dsc* desc)
 
 	MAKE_desc(dsqlScratch, &desc1, expr);
 	MAKE_desc(dsqlScratch, &desc2, start);
-	MAKE_desc(dsqlScratch, &desc3, length);
+	if (length)
+		MAKE_desc(dsqlScratch, &desc3, length);
 
-	DSqlDataTypeUtil(dsqlScratch).makeSubstr(desc, &desc1, &desc2, &desc3);
+	DSqlDataTypeUtil(dsqlScratch).makeSubstr(desc, &desc1, &desc2, (length ? &desc3 : NULL));
 }
 
 void SubstringNode::getDesc(thread_db* tdbb, CompilerScratch* csb, dsc* desc)
diff --git a/src/dsql/parse.y b/src/dsql/parse.y
index 40ab39e0e0..0a94a6a114 100644
--- a/src/dsql/parse.y
+++ b/src/dsql/parse.y
@@ -101,7 +101,6 @@
 const long SHRT_POS_MAX			= 32767;
 const long SHRT_UNSIGNED_MAX	= 65535;
 const long SHRT_NEG_MAX			= 32768;
-const long LONG_POS_MAX			= 2147483647;
 const int POSITIVE	= 0;
 const int NEGATIVE	= 1;
 const int UNSIGNED	= 2;
@@ -6255,7 +6254,7 @@ substring_function
 
 %type <valueExprNode> string_length_opt
 string_length_opt
-	: /* nothing */		{ $$ = MAKE_const_slong(LONG_POS_MAX); }
+	: /* nothing */		{ $$ = NULL; }
 	| FOR value 		{ $$ = $2; }
 	;
 
diff --git a/src/jrd/DataTypeUtil.cpp b/src/jrd/DataTypeUtil.cpp
index f085f68da5..5b2b6f41ee 100644
--- a/src/jrd/DataTypeUtil.cpp
+++ b/src/jrd/DataTypeUtil.cpp
@@ -28,6 +28,7 @@
 #include "../jrd/DataTypeUtil.h"
 #include "../jrd/SysFunction.h"
 #include "../jrd/align.h"
+#include "../common/cvt.h"
 #include "../common/dsc.h"
 #include "../jrd/ibase.h"
 #include "../jrd/intl.h"
@@ -300,10 +301,17 @@ void DataTypeUtilBase::makeSubstr(dsc* result, const dsc* value, const dsc* offs
 	}
 
 	result->setTextType(value->getTextType());
-	result->setNullable(value->isNullable() || offset->isNullable() || length->isNullable());
+	result->setNullable(value->isNullable() || offset->isNullable() || (length && length->isNullable()));
 
 	if (result->isText())
-		result->dsc_length = fixLength(result, convertLength(value, result)) + sizeof(USHORT);
+	{
+		SLONG len = convertLength(value, result);
+
+		if (length && length->dsc_address)	// constant
+			len = MIN(len, CVT_get_long(length, 0, ERR_post) * maxBytesPerChar(result->getCharSet()));
+
+		result->dsc_length = fixLength(result, len) + sizeof(USHORT);
+	}
 }