IssueTracker

What you expected to happen:

The documented restriction of ORDER BY in CREATE SEARCH INDEX is only to be INT64, but it seems that there is the undocumented restriction related with PENDING_COMMIT_TIMESTAMP()

https://cloud.google.com/spanner/docs/reference/standard-sql/data-definition-language#create-search-index

A list of INT64 columns that the search index will store rows in that order within a partition. The column must be NOT NULL, or the index must define WHERE IS NOT NULL. It can be one or two columns when provided. When two columns are provided, disable_automatic_uid_column must be set to true.

Steps to reproduce:

spanner> CREATE TABLE FTSTest(
           PK INT64 PRIMARY KEY,
           Text STRING(MAX),
           Text_Tokens TOKENLIST AS (TOKENIZE_FULLTEXT(Text)) HIDDEN,
           CreatedAt TIMESTAMP NOT NULL OPTIONS (allow_commit_timestamp=true),
           CreatedAtMillis INT64 AS (UNIX_MILLIS(createdAt)),
         );
Query OK, 0 rows affected (10.15 sec)

# Without SEARCH INDEX, it succeeded.
spanner> INSERT INTO FTSTest(PK, CreatedAt) VALUES(1, PENDING_COMMIT_TIMESTAMP());
Query OK, 1 rows affected (0.83 sec)

spanner> CREATE SEARCH INDEX FTSTestSearchIndex ON FTSTest(
           Text_Tokens
         )
         ORDER BY CreatedAtMillis
         WHERE CreatedAtMillis IS NOT NULL;
Query OK, 0 rows affected (38.56 sec)

# Same INSERT has failed with SEARCH INDEX
spanner> INSERT INTO FTSTest(PK, CreatedAt) VALUES(2, PENDING_COMMIT_TIMESTAMP());
ERROR: transaction was aborted: spanner: code = "FailedPrecondition", desc = "Expected TIMESTAMP for column FTSTest.CreatedAt. Got <internal> whose type is INT64."

# Succeeded if PENDING_COMMIT_TIMESTAMP() is not used.
spanner> INSERT INTO FTSTest(PK, CreatedAt) VALUES(2, CURRENT_TIMESTAMP());
Query OK, 1 rows affected (0.54 sec)

Other information (workarounds you have tried, documentation consulted, etc):

For workaround, we need to give up to use commit timestamp.