builtins: support current_timestamp with precision

Release note (sql change): This PR adds new functionality that allows
users to use CURRENT_TIMESTAMP with a given precision from 0-6, e.g.
`SELECT CURRENT_TIMESTAMP(4)`.

docs/generated/sql/bnf/stmt_block.bnf

@@ -2304,6 +2304,7 @@ special_function ::=
 	'CURRENT_DATE' '(' ')'
 	| 'CURRENT_SCHEMA' '(' ')'
 	| 'CURRENT_TIMESTAMP' '(' ')'
+	| 'CURRENT_TIMESTAMP' '(' a_expr ')'
 	| 'CURRENT_USER' '(' ')'
 	| 'EXTRACT' '(' extract_list ')'
 	| 'EXTRACT_DURATION' '(' extract_list ')'

docs/generated/sql/functions.md

@@ -312,6 +312,21 @@ has no relationship with the commit order of concurrent transactions.</p>
 and which stays constant throughout the transaction. This timestamp
 has no relationship with the commit order of concurrent transactions.</p>
 </span></td></tr>
+<tr><td><a name="current_timestamp"></a><code>current_timestamp(precision: <a href="int.html">int</a>) &rarr; <a href="date.html">date</a></code></td><td><span class="funcdesc"><p>Returns the time of the current transaction.</p>
+<p>The value is based on a timestamp picked when the transaction starts
+and which stays constant throughout the transaction. This timestamp
+has no relationship with the commit order of concurrent transactions.</p>
+</span></td></tr>
+<tr><td><a name="current_timestamp"></a><code>current_timestamp(precision: <a href="int.html">int</a>) &rarr; <a href="timestamp.html">timestamp</a></code></td><td><span class="funcdesc"><p>Returns the time of the current transaction.</p>
+<p>The value is based on a timestamp picked when the transaction starts
+and which stays constant throughout the transaction. This timestamp
+has no relationship with the commit order of concurrent transactions.</p>
+</span></td></tr>
+<tr><td><a name="current_timestamp"></a><code>current_timestamp(precision: <a href="int.html">int</a>) &rarr; <a href="timestamp.html">timestamptz</a></code></td><td><span class="funcdesc"><p>Returns the time of the current transaction.</p>
+<p>The value is based on a timestamp picked when the transaction starts
+and which stays constant throughout the transaction. This timestamp
+has no relationship with the commit order of concurrent transactions.</p>
+</span></td></tr>
 <tr><td><a name="date_trunc"></a><code>date_trunc(element: <a href="string.html">string</a>, input: <a href="date.html">date</a>) &rarr; <a href="timestamp.html">timestamptz</a></code></td><td><span class="funcdesc"><p>Truncates <code>input</code> to precision <code>element</code>.  Sets all fields that are less
 significant than <code>element</code> to zero (or one, for day and month)</p>
 <p>Compatible elements: millennium, century, decade, year, quarter, month,

pkg/sql/logictest/testdata/logic_test/timestamp

@@ -74,6 +74,11 @@ select '1-1-18 1:00:00.001-8':::TIMESTAMPTZ(6)
 ----
 2001-01-18 01:00:00.001 -0800 PST
 
+query B
+select current_timestamp(3) - current_timestamp <= '1ms'::interval
+----
+true
+
 statement ok
 CREATE TABLE timestamp_test (
   id integer PRIMARY KEY,

pkg/sql/parser/parse_test.go

@@ -1553,6 +1553,8 @@ func TestParse2(t *testing.T) {
 			`SELECT current_database()`},
 		{`SELECT CURRENT_TIMESTAMP`,
 			`SELECT current_timestamp()`},
+		{`SELECT current_timestamp(6)`,
+			`SELECT current_timestamp(6)`},
 		{`SELECT CURRENT_DATE`,
 			`SELECT current_date()`},
 		{`SELECT POSITION(a IN b)`,

pkg/sql/parser/sql.y

@@ -8238,6 +8238,10 @@ special_function:
   {
     $$.val = &tree.FuncExpr{Func: tree.WrapFunction($1)}
   }
+| CURRENT_TIMESTAMP '(' a_expr ')'
+  {
+    $$.val = &tree.FuncExpr{Func: tree.WrapFunction($1), Exprs: tree.Exprs{$3.expr()}}
+  }
 | CURRENT_TIMESTAMP '(' error { return helpWithFunctionByName(sqllex, $1) }
 | CURRENT_TIME '(' ')'
   {

pkg/sql/sem/builtins/builtins.go

@@ -1675,7 +1675,7 @@ CockroachDB supports the following flags:
 	),
 
 	"now":                   txnTSImpl,
-	"current_timestamp":     txnTSImpl,
+	"current_timestamp":     txnTSWithPrecisionImpl,
 	"transaction_timestamp": txnTSImpl,
 
 	"statement_timestamp": makeBuiltin(
@@ -3496,12 +3496,8 @@ has no relationship with the commit order of concurrent transactions.`
 
 var txnTSDoc = `Returns the time of the current transaction.` + txnTSContextDoc
 
-var txnTSImpl = makeBuiltin(
-	tree.FunctionProperties{
-		Category: categoryDateAndTime,
-		Impure:   true,
-	},
-	tree.Overload{
+var txnTSOverloads = []tree.Overload{
+	{
 		Types:             tree.ArgTypes{},
 		ReturnType:        tree.FixedReturnType(types.TimestampTZ),
 		PreferredOverload: true,
@@ -3510,20 +3506,79 @@ var txnTSImpl = makeBuiltin(
 		},
 		Info: txnTSDoc,
 	},
-	tree.Overload{
+	{
 		Types:      tree.ArgTypes{},
 		ReturnType: tree.FixedReturnType(types.Timestamp),
 		Fn: func(ctx *tree.EvalContext, args tree.Datums) (tree.Datum, error) {
 			return ctx.GetTxnTimestampNoZone(time.Microsecond), nil
 		},
 		Info: txnTSDoc,
 	},
-	tree.Overload{
+	{
 		Types:      tree.ArgTypes{},
 		ReturnType: tree.FixedReturnType(types.Date),
 		Fn:         currentDate,
 		Info:       txnTSDoc,
 	},
+}
+
+var txnTSWithPrecisionOverloads = append(
+	[]tree.Overload{
+		{
+			Types:             tree.ArgTypes{{"precision", types.Int}},
+			ReturnType:        tree.FixedReturnType(types.TimestampTZ),
+			PreferredOverload: true,
+			Fn: func(ctx *tree.EvalContext, args tree.Datums) (tree.Datum, error) {
+				prec := int32(tree.MustBeDInt(args[0]))
+				if prec < 0 || prec > 6 {
+					return nil, pgerror.Newf(pgcode.NumericValueOutOfRange, "precision %d out of range", prec)
+				}
+				return ctx.GetTxnTimestamp(tree.TimeFamilyPrecisionToRoundDuration(prec)), nil
+			},
+			Info: txnTSDoc,
+		},
+		{
+			Types:      tree.ArgTypes{{"precision", types.Int}},
+			ReturnType: tree.FixedReturnType(types.Timestamp),
+			Fn: func(ctx *tree.EvalContext, args tree.Datums) (tree.Datum, error) {
+				prec := int32(tree.MustBeDInt(args[0]))
+				if prec < 0 || prec > 6 {
+					return nil, pgerror.Newf(pgcode.NumericValueOutOfRange, "precision %d out of range", prec)
+				}
+				return ctx.GetTxnTimestampNoZone(tree.TimeFamilyPrecisionToRoundDuration(prec)), nil
+			},
+			Info: txnTSDoc,
+		},
+		{
+			Types:      tree.ArgTypes{{"precision", types.Int}},
+			ReturnType: tree.FixedReturnType(types.Date),
+			Fn: func(ctx *tree.EvalContext, args tree.Datums) (tree.Datum, error) {
+				prec := int32(tree.MustBeDInt(args[0]))
+				if prec < 0 || prec > 6 {
+					return nil, pgerror.Newf(pgcode.NumericValueOutOfRange, "precision %d out of range", prec)
+				}
+				return currentDate(ctx, args)
+			},
+			Info: txnTSDoc,
+		},
+	},
+	txnTSOverloads...,
+)
+
+var txnTSImpl = makeBuiltin(
+	tree.FunctionProperties{
+		Category: categoryDateAndTime,
+		Impure:   true,
+	},
+	txnTSOverloads...,
+)
+
+var txnTSWithPrecisionImpl = makeBuiltin(
+	tree.FunctionProperties{
+		Category: categoryDateAndTime,
+		Impure:   true,
+	},
+	txnTSWithPrecisionOverloads...,
 )
 
 func currentDate(ctx *tree.EvalContext, args tree.Datums) (tree.Datum, error) {

pkg/sql/sem/tree/normalize_test.go

@@ -183,6 +183,7 @@ func TestNormalizeExpr(t *testing.T) {
 		{`clock_timestamp()`, `clock_timestamp()`},
 		{`now()`, `now()`},
 		{`current_timestamp()`, `current_timestamp()`},
+		{`current_timestamp(5)`, `current_timestamp(5)`},
 		{`transaction_timestamp()`, `transaction_timestamp()`},
 		{`statement_timestamp()`, `statement_timestamp()`},
 		{`cluster_logical_timestamp()`, `cluster_logical_timestamp()`},