sql: add parse_timestamp builtin

Add a builtin that can be used to parse timestamp strings. This is
like a cast, but it does not accept relative timestamps so it can be
immutable.

Only immutable expressions are allowed in computed column expressions
or partial index predicates; unlike casts, the new function can be
used in such expressions.

Fixes cockroachdb#60578.

Release notes (sql change): A new parse_timestamp function can be used
to parse absolute timestamp strings in computed column expressions or
partial index predicates.

docs/generated/sql/functions.md

@@ -2371,6 +2371,8 @@ to recreate a database.’</p>
 </span></td></tr>
 <tr><td><a name="overlay"></a><code>overlay(input: <a href="string.html">string</a>, overlay_val: <a href="string.html">string</a>, start_pos: <a href="int.html">int</a>, end_pos: <a href="int.html">int</a>) &rarr; <a href="string.html">string</a></code></td><td><span class="funcdesc"><p>Deletes the characters in <code>input</code> between <code>start_pos</code> and <code>end_pos</code> (count starts at 1), and then insert <code>overlay_val</code> at <code>start_pos</code>.</p>
 </span></td></tr>
+<tr><td><a name="parse_timestamp"></a><code>parse_timestamp(string: <a href="string.html">string</a>) &rarr; <a href="timestamp.html">timestamp</a></code></td><td><span class="funcdesc"><p>Convert a string containing an absolute timestamp to the corresponding timestamp.</p>
+</span></td></tr>
 <tr><td><a name="pg_collation_for"></a><code>pg_collation_for(str: anyelement) &rarr; <a href="string.html">string</a></code></td><td><span class="funcdesc"><p>Returns the collation of the argument</p>
 </span></td></tr>
 <tr><td><a name="quote_ident"></a><code>quote_ident(val: <a href="string.html">string</a>) &rarr; <a href="string.html">string</a></code></td><td><span class="funcdesc"><p>Return <code>val</code> suitably quoted to serve as identifier in a SQL statement.</p>

pkg/sql/logictest/testdata/logic_test/datetime

@@ -1703,3 +1703,56 @@ query TT
 SELECT 'infinity'::timestamp, '-infinity'::timestamptz
 ----
 294276-12-31 23:59:59.999999 +0000 +0000  -4713-11-24 00:00:00 +0000 +0000
+
+query T
+SELECT parse_timestamp('2020-01-02 01:02:03')
+----
+2020-01-02 01:02:03 +0000 +0000
+
+query error could not parse
+SELECT parse_timestamp('foo')
+
+query error parse_timestamp\(\): relative timestamps are not supported
+SELECT parse_timestamp('now')
+
+query error parse_timestamp\(\): relative timestamps are not supported
+SELECT parse_timestamp('tomorrow')
+
+# Verify that parse_timestamp can be used in computed column expressions.
+statement ok
+CREATE TABLE timestamps (s STRING, ts TIMESTAMP AS (parse_timestamp(s)) STORED)
+
+query error parse_timestamp\(\): relative timestamps are not supported
+INSERT INTO timestamps VALUES ('tomorrow')
+
+statement ok
+INSERT INTO timestamps VALUES ('2020-01-02 01:02:03'), ('2015-08-25 04:45:45.53453+01:00'), (NULL)
+
+query TT colnames
+SELECT * FROM timestamps ORDER BY s
+----
+s                                ts
+NULL                             NULL
+2015-08-25 04:45:45.53453+01:00  2015-08-25 04:45:45.53453 +0000 +0000
+2020-01-02 01:02:03              2020-01-02 01:02:03 +0000 +0000
+
+statement ok
+SET TIME ZONE 'America/New_York'
+
+# Insert the same values again (stored columns are computed on insert).
+statement ok
+INSERT INTO timestamps VALUES ('2020-01-02 01:02:03'), ('2015-08-25 04:45:45.53453+01:00'), (NULL)
+
+query TT colnames
+SELECT * FROM timestamps ORDER BY s
+----
+s                                ts
+NULL                             NULL
+NULL                             NULL
+2015-08-25 04:45:45.53453+01:00  2015-08-25 04:45:45.53453 +0000 +0000
+2015-08-25 04:45:45.53453+01:00  2015-08-25 04:45:45.53453 +0000 +0000
+2020-01-02 01:02:03              2020-01-02 01:02:03 +0000 +0000
+2020-01-02 01:02:03              2020-01-02 01:02:03 +0000 +0000
+
+statement ok
+RESET TIME ZONE

pkg/sql/sem/builtins/builtins.go

@@ -2734,6 +2734,31 @@ may increase either contention or retry errors, or both.`,
 		},
 	),
 
+	// parse_timestamp converts strings to timestamps. It is useful in expressions
+	// where casts (which are not immutable) cannot be used, like computed column
+	// expressions or partial index predicates. Only absolute timestamps that do
+	// not depend on the current context are supported (relative timestamps like
+	// 'now' are not supported).
+	"parse_timestamp": makeBuiltin(defProps(),
+		tree.Overload{
+			Types:      tree.ArgTypes{{"string", types.String}},
+			ReturnType: tree.FixedReturnType(types.Timestamp),
+			Fn: func(ctx *tree.EvalContext, args tree.Datums) (tree.Datum, error) {
+				arg := string(tree.MustBeDString(args[0]))
+				ts, dependsOnContext, err := tree.ParseDTimestamp(ctx, arg, time.Microsecond)
+				if err != nil {
+					return nil, err
+				}
+				if dependsOnContext {
+					return nil, pgerror.Newf(pgcode.InvalidParameterValue, "relative timestamps are not supported")
+				}
+				return ts, nil
+			},
+			Info:       "Convert a string containing an absolute timestamp to the corresponding timestamp.",
+			Volatility: tree.VolatilityImmutable,
+		},
+	),
+
 	// Array functions.
 
 	"string_to_array": makeBuiltin(arrayPropsNullableArgs(),