Commit timestamps in PostgreSQL-dialect databases

This page describes how to write a commit timestamp for each insert and update operation that you perform with Spanner in PostgreSQL-dialect databases.

Insert commit timestamps

The commit timestamp, based on TrueTime technology, is the time when a transaction is committed in the database. You can atomically store the commit timestamp of a transaction into a column. Using the commit timestamps stored in tables, you can determine the exact ordering of mutations and build features like changelogs.

To insert commit timestamps in your database, complete the following steps:

  1. Create a column of type SPANNER.COMMIT_TIMESTAMP. For example:

    CREATETABLEPerformances(...LastUpdateTimeSPANNER.COMMIT_TIMESTAMPNOTNULL,...PRIMARYKEY(...));

  2. If you are performing inserts or updates with DML, use the SPANNER.PENDING_COMMIT_TIMESTAMP() function to write the commit timestamp.

    If you are performing inserts or updates with prepared statements or mutations, use the placeholder string SPANNER.COMMIT_TIMESTAMP() for your commit timestamp column. You can also use the commit timestamp constant provided by the client library. For example, this constant in the Java client is Value.COMMIT_TIMESTAMP.

When Spanner commits the transaction by using these placeholders as column values, the actual commit timestamp is written to the specified column. You can then use this column value to create a history of updates to the table.

Commit timestamp values are not guaranteed to be unique. Transactions that write to non-overlapping sets of fields might have the same timestamp. Transactions that write to overlapping sets of fields have unique timestamps.

Spanner commit timestamps have microsecond granularity, and they are converted to nanoseconds when stored in SPANNER.COMMIT_TIMESTAMP columns.

Keys and indexes

You can use a commit timestamp column as a primary key column or as a non-key column. Primary keys can be defined as ASC or DESC.

  • ASC (default) - Ascending keys are ideal for answering queries from a specific time forward.
  • DESC - Descending keys keep the latest rows at the top of the table. They provide quick access to the latest records.

Avoid hotspots

Using commit timestamps under the following scenarios creates hotspots, which reduce data performance:

  • Commit timestamp column as the first part of the primary key of a table.

    CREATE TABLE Users ( LastAccess SPANNER.COMMIT_TIMESTAMP NOT NULL, UserId bigint NOT NULL, ... PRIMARY KEY (LastAccess, UserId) ) ;

  • Commit timestamp primary key column as the first part of a secondary index.

    CREATE INDEX UsersByLastAccess ON Users(LastAccess)

    or

    CREATE INDEX UsersByLastAccessAndName ON Users(LastAccess, FirstName)

Hotspots reduce data performance, even with low write rates. There is no performance overhead if commit timestamps are enabled on non-key columns that are not indexed.

Add a commit timestamp column to an existing table

To add a commit timestamp column to an existing table, use the ALTER TABLE statement. For example to add a LastUpdateTime column to the Performances table, use the following statement:

ALTERTABLEPerformancesADDCOLUMNLastUpdateTimeSPANNER.COMMIT_TIMESTAMP;

Write a commit timestamp using a DML statement

You use the SPANNER.PENDING_COMMIT_TIMESTAMP() function to write the commit timestamp in a DML statement. Spanner selects the commit timestamp when the transaction commits.

The following DML statement updates the LastUpdateTime column in the Performances table with the commit timestamp:

UPDATEPerformancesSETLastUpdateTime=SPANNER.PENDING_COMMIT_TIMESTAMP()WHERESingerId=1ANDVenueId=2ANDEventDate="2015-10-21"

Insert a row using a mutation

When inserting a row, Spanner writes the commit timestamp value only if you include the column in the column list and pass the spanner.commit_timestamp() placeholder string (or client library constant) as its value. For example:

C++

voidInsertDataWithTimestamp(google::cloud::spanner::Clientclient){namespacespanner=::google::cloud::spanner;autocommit_result=client.Commit(spanner::Mutations{spanner::InsertOrUpdateMutationBuilder("Performances",{"SingerId","VenueId","EventDate","Revenue","LastUpdateTime"}).EmplaceRow(1,4,absl::CivilDay(2017,10,5),11000,spanner::CommitTimestamp{}).EmplaceRow(1,19,absl::CivilDay(2017,11,2),15000,spanner::CommitTimestamp{}).EmplaceRow(2,42,absl::CivilDay(2017,12,23),7000,spanner::CommitTimestamp{}).Build()});if(!commit_result)throwstd::move(commit_result).status();std::cout << "Update was successful [spanner_insert_data_with_timestamp_column]\n";}

C#

usingGoogle.Cloud.Spanner.Data;usingSystem;usingSystem.Collections.Generic;usingSystem.Linq;usingSystem.Threading.Tasks;publicclassWriteDataWithTimestampAsyncSample{publicclassPerformance{publicintSingerId{get;set;}publicintVenueId{get;set;}publicDateTimeEventDate{get;set;}publiclongRevenue{get;set;}}publicasyncTask<int>WriteDataWithTimestampAsync(stringprojectId,stringinstanceId,stringdatabaseId){stringconnectionString=$"Data Source=projects/{projectId}/instances/{instanceId}/databases/{databaseId}";List<Performance>performances=newList<Performance>{newPerformance{SingerId=1,VenueId=4,EventDate=DateTime.Parse("2017-10-05"),Revenue=11000},newPerformance{SingerId=1,VenueId=19,EventDate=DateTime.Parse("2017-11-02"),Revenue=15000},newPerformance{SingerId=2,VenueId=42,EventDate=DateTime.Parse("2017-12-23"),Revenue=7000},};// Create connection to Cloud Spanner.usingvarconnection=newSpannerConnection(connectionString);awaitconnection.OpenAsync();// Insert rows into the Performances table.varrowCountAarray=awaitTask.WhenAll(performances.Select(performance=> {varcmd=connection.CreateInsertCommand("Performances",newSpannerParameterCollection{{"SingerId",SpannerDbType.Int64,performance.SingerId},{"VenueId",SpannerDbType.Int64,performance.VenueId},{"EventDate",SpannerDbType.Date,performance.EventDate},{"Revenue",SpannerDbType.Int64,performance.Revenue},{"LastUpdateTime",SpannerDbType.Timestamp,SpannerParameter.CommitTimestamp},});returncmd.ExecuteNonQueryAsync();}));returnrowCountAarray.Sum();}}

Go

import("context""cloud.google.com/go/spanner")funcwriteWithTimestamp(dbstring)error{ctx:=context.Background()client,err:=spanner.NewClient(ctx,db)iferr!=nil{returnerr}deferclient.Close()performanceColumns:=[]string{"SingerId","VenueId","EventDate","Revenue","LastUpdateTime"}m:=[]*spanner.Mutation{spanner.InsertOrUpdate("Performances",performanceColumns,[]interface{}{1,4,"2017-10-05",11000,spanner.CommitTimestamp}),spanner.InsertOrUpdate("Performances",performanceColumns,[]interface{}{1,19,"2017-11-02",15000,spanner.CommitTimestamp}),spanner.InsertOrUpdate("Performances",performanceColumns,[]interface{}{2,42,"2017-12-23",7000,spanner.CommitTimestamp}),}_,err=client.Apply(ctx,m)returnerr}

Java

staticfinalList<Performance>PERFORMANCES=Arrays.asList(newPerformance(1,4,"2017-10-05",11000),newPerformance(1,19,"2017-11-02",15000),newPerformance(2,42,"2017-12-23",7000));staticvoidwriteExampleDataWithTimestamp(DatabaseClientdbClient){List<Mutation>mutations=newArrayList<>();for(Performanceperformance:PERFORMANCES){mutations.add(Mutation.newInsertBuilder("Performances").set("SingerId").to(performance.singerId).set("VenueId").to(performance.venueId).set("EventDate").to(performance.eventDate).set("Revenue").to(performance.revenue).set("LastUpdateTime").to(Value.COMMIT_TIMESTAMP).build());}dbClient.write(mutations);}

Node.js

// Imports the Google Cloud client libraryconst{Spanner}=require('@google-cloud/spanner');/** * TODO(developer): Uncomment the following lines before running the sample. */// const projectId = 'my-project-id';// const instanceId = 'my-instance';// const databaseId = 'my-database';// Creates a clientconstspanner=newSpanner({projectId:projectId,});// Gets a reference to a Cloud Spanner instance and databaseconstinstance=spanner.instance(instanceId);constdatabase=instance.database(databaseId);// Instantiate Spanner table objectsconstperformancesTable=database.table('Performances');constdata=[{SingerId:'1',VenueId:'4',EventDate:'2017-10-05',Revenue:'11000',LastUpdateTime:'spanner.commit_timestamp()',},{SingerId:'1',VenueId:'19',EventDate:'2017-11-02',Revenue:'15000',LastUpdateTime:'spanner.commit_timestamp()',},{SingerId:'2',VenueId:'42',EventDate:'2017-12-23',Revenue:'7000',LastUpdateTime:'spanner.commit_timestamp()',},];// Inserts rows into the Singers table// Note: Cloud Spanner interprets Node.js numbers as FLOAT64s, so// they must be converted to strings before being inserted as INT64stry{awaitperformancesTable.insert(data);console.log('Inserted data.');}catch(err){console.error('ERROR:',err);}finally{// Close the database when finisheddatabase.close();}

PHP

use Google\Cloud\Spanner\SpannerClient;/** * Inserts sample data into a table with a commit timestamp column. * * The database and table must already exist and can be created using * `create_table_with_timestamp_column`. * Example: * ``` * insert_data_with_timestamp_column($instanceId, $databaseId); * ``` * * @param string $instanceId The Spanner instance ID. * @param string $databaseId The Spanner database ID. */function insert_data_with_timestamp_column(string $instanceId, string $databaseId): void{ $spanner = new SpannerClient(); $instance = $spanner->instance($instanceId); $database = $instance->database($databaseId); $operation = $database->transaction(['singleUse' => true]) ->insertBatch('Performances', [ ['SingerId' => 1, 'VenueId' => 4, 'EventDate' => '2017-10-05', 'Revenue' => 11000, 'LastUpdateTime' => $spanner->commitTimestamp()], ['SingerId' => 1, 'VenueId' => 19, 'EventDate' => '2017-11-02', 'Revenue' => 15000, 'LastUpdateTime' => $spanner->commitTimestamp()], ['SingerId' => 2, 'VenueId' => 42, 'EventDate' => '2017-12-23', 'Revenue' => 7000, 'LastUpdateTime' => $spanner->commitTimestamp()], ]) ->commit(); print('Inserted data.' . PHP_EOL);}

Python

definsert_data_with_timestamp(instance_id,database_id):"""Inserts data with a COMMIT_TIMESTAMP field into a table."""spanner_client=spanner.Client()instance=spanner_client.instance(instance_id)database=instance.database(database_id)withdatabase.batch()asbatch:batch.insert(table="Performances",columns=("SingerId","VenueId","EventDate","Revenue","LastUpdateTime"),values=[(1,4,"2017-10-05",11000,spanner.COMMIT_TIMESTAMP),(1,19,"2017-11-02",15000,spanner.COMMIT_TIMESTAMP),(2,42,"2017-12-23",7000,spanner.COMMIT_TIMESTAMP),],)print("Inserted data.")

Ruby

# project_id = "Your Google Cloud project ID"# instance_id = "Your Spanner instance ID"# database_id = "Your Spanner database ID"require"google/cloud/spanner"spanner=Google::Cloud::Spanner.newproject:project_idclient=spanner.clientinstance_id,database_id# Get commit_timestampcommit_timestamp=client.commit_timestampclient.commitdo|c|c.insert"Performances",[{SingerId:1,VenueId:4,EventDate:"2017-10-05",Revenue:11_000,LastUpdateTime:commit_timestamp},{SingerId:1,VenueId:19,EventDate:"2017-11-02",Revenue:15_000,LastUpdateTime:commit_timestamp},{SingerId:2,VenueId:42,EventDate:"2017-12-23",Revenue:7000,LastUpdateTime:commit_timestamp}]endputs"Inserted data"

If you have mutations on rows in multiple tables, you must specify spanner.commit_timestamp() (or client library constant) for the commit timestamp column in each table.

Update a row using a mutation

When updating a row, Spanner writes the commit timestamp value only if you include the column in the column list and pass the spanner.commit_timestamp() placeholder string (or client library constant) as its value. You cannot update the primary key of a row. To update the primary key, delete the existing row and create a new row.

For example, to update a commit timestamp column named LastUpdateTime:

C++

voidUpdateDataWithTimestamp(google::cloud::spanner::Clientclient){namespacespanner=::google::cloud::spanner;autocommit_result=client.Commit(spanner::Mutations{spanner::UpdateMutationBuilder("Albums",{"SingerId","AlbumId","MarketingBudget","LastUpdateTime"}).EmplaceRow(1,1,1000000,spanner::CommitTimestamp{}).EmplaceRow(2,2,750000,spanner::CommitTimestamp{}).Build()});if(!commit_result)throwstd::move(commit_result).status();std::cout << "Update was successful [spanner_update_data_with_timestamp_column]\n";}

C#

usingGoogle.Cloud.Spanner.Data;usingSystem;usingSystem.Threading.Tasks;publicclassUpdateDataWithTimestampColumnAsyncSample{publicasyncTask<int>UpdateDataWithTimestampColumnAsync(stringprojectId,stringinstanceId,stringdatabaseId){stringconnectionString=$"Data Source=projects/{projectId}/instances/{instanceId}/databases/{databaseId}";usingvarconnection=newSpannerConnection(connectionString);varrowCount=0;usingvarupdateCmd1=connection.CreateUpdateCommand("Albums",newSpannerParameterCollection{{"SingerId",SpannerDbType.Int64,1},{"AlbumId",SpannerDbType.Int64,1},{"MarketingBudget",SpannerDbType.Int64,1000000},{"LastUpdateTime",SpannerDbType.Timestamp,SpannerParameter.CommitTimestamp},});rowCount+=awaitupdateCmd1.ExecuteNonQueryAsync();usingvarupdateCmd2=connection.CreateUpdateCommand("Albums",newSpannerParameterCollection{{"SingerId",SpannerDbType.Int64,2},{"AlbumId",SpannerDbType.Int64,2},{"MarketingBudget",SpannerDbType.Int64,750000},{"LastUpdateTime",SpannerDbType.Timestamp,SpannerParameter.CommitTimestamp},});rowCount+=awaitupdateCmd2.ExecuteNonQueryAsync();Console.WriteLine("Updated data.");returnrowCount;}}

Go

import("context""io""cloud.google.com/go/spanner")funcupdateWithTimestamp(wio.Writer,dbstring)error{ctx:=context.Background()client,err:=spanner.NewClient(ctx,db)iferr!=nil{returnerr}deferclient.Close()cols:=[]string{"SingerId","AlbumId","MarketingBudget","LastUpdateTime"}_,err=client.Apply(ctx,[]*spanner.Mutation{spanner.Update("Albums",cols,[]interface{}{1,1,1000000,spanner.CommitTimestamp}),spanner.Update("Albums",cols,[]interface{}{2,2,750000,spanner.CommitTimestamp}),})returnerr}

Java

staticvoidupdateWithTimestamp(DatabaseClientdbClient){// Mutation can be used to update/insert/delete a single row in a table. Here we use// newUpdateBuilder to create update mutations.List<Mutation>mutations=Arrays.asList(Mutation.newUpdateBuilder("Albums").set("SingerId").to(1).set("AlbumId").to(1).set("MarketingBudget").to(1000000).set("LastUpdateTime").to(Value.COMMIT_TIMESTAMP).build(),Mutation.newUpdateBuilder("Albums").set("SingerId").to(2).set("AlbumId").to(2).set("MarketingBudget").to(750000).set("LastUpdateTime").to(Value.COMMIT_TIMESTAMP).build());// This writes all the mutations to Cloud Spanner atomically.dbClient.write(mutations);}

Node.js

// ...// Imports the Google Cloud client libraryconst{Spanner}=require('@google-cloud/spanner');/** * TODO(developer): Uncomment the following lines before running the sample. */// const projectId = 'my-project-id';// const instanceId = 'my-instance';// const databaseId = 'my-database';// Creates a clientconstspanner=newSpanner({projectId:projectId,});// Gets a reference to a Cloud Spanner instance and databaseconstinstance=spanner.instance(instanceId);constdatabase=instance.database(databaseId);// Update a row in the Albums table// Note: Cloud Spanner interprets Node.js numbers as FLOAT64s, so they// must be converted to strings before being inserted as INT64sconstalbumsTable=database.table('Albums');constdata=[{SingerId:'1',AlbumId:'1',MarketingBudget:'1000000',LastUpdateTime:'spanner.commit_timestamp()',},{SingerId:'2',AlbumId:'2',MarketingBudget:'750000',LastUpdateTime:'spanner.commit_timestamp()',},];try{awaitalbumsTable.update(data);console.log('Updated data.');}catch(err){console.error('ERROR:',err);}finally{// Close the database when finisheddatabase.close();}

PHP

use Google\Cloud\Spanner\SpannerClient;/** * Updates sample data in a table with a commit timestamp column. * * Before executing this method, a new column MarketingBudget has to be added to the Albums * table by applying the DDL statement "ALTER TABLE Albums ADD COLUMN MarketingBudget INT64". * * In addition this update expects the LastUpdateTime column added by applying the DDL statement * "ALTER TABLE Albums ADD COLUMN LastUpdateTime TIMESTAMP OPTIONS (allow_commit_timestamp=true)" * * Example: * ``` * update_data_with_timestamp_column($instanceId, $databaseId); * ``` * * @param string $instanceId The Spanner instance ID. * @param string $databaseId The Spanner database ID. */function update_data_with_timestamp_column(string $instanceId, string $databaseId): void{ $spanner = new SpannerClient(); $instance = $spanner->instance($instanceId); $database = $instance->database($databaseId); $operation = $database->transaction(['singleUse' => true]) ->updateBatch('Albums', [ ['SingerId' => 1, 'AlbumId' => 1, 'MarketingBudget' => 1000000, 'LastUpdateTime' => $spanner->commitTimestamp()], ['SingerId' => 2, 'AlbumId' => 2, 'MarketingBudget' => 750000, 'LastUpdateTime' => $spanner->commitTimestamp()], ]) ->commit(); print('Updated data.' . PHP_EOL);}

Python

defupdate_data_with_timestamp(instance_id,database_id):"""Updates Performances tables in the database with the COMMIT_TIMESTAMP column. This updates the `MarketingBudget` column which must be created before running this sample. You can add the column by running the `add_column` sample or by running this DDL statement against your database: ALTER TABLE Albums ADD COLUMN MarketingBudget INT64 In addition this update expects the LastUpdateTime column added by applying this DDL statement against your database: ALTER TABLE Albums ADD COLUMN LastUpdateTime TIMESTAMP OPTIONS(allow_commit_timestamp=true) """spanner_client=spanner.Client()instance=spanner_client.instance(instance_id)database=instance.database(database_id)withdatabase.batch()asbatch:batch.update(table="Albums",columns=("SingerId","AlbumId","MarketingBudget","LastUpdateTime"),values=[(1,1,1000000,spanner.COMMIT_TIMESTAMP),(2,2,750000,spanner.COMMIT_TIMESTAMP),],)print("Updated data.")

Ruby

# project_id = "Your Google Cloud project ID"# instance_id = "Your Spanner instance ID"# database_id = "Your Spanner database ID"require"google/cloud/spanner"spanner=Google::Cloud::Spanner.newproject:project_idclient=spanner.clientinstance_id,database_idcommit_timestamp=client.commit_timestampclient.commitdo|c|c.update"Albums",[{SingerId:1,AlbumId:1,MarketingBudget:100_000,LastUpdateTime:commit_timestamp},{SingerId:2,AlbumId:2,MarketingBudget:750_000,LastUpdateTime:commit_timestamp}]endputs"Updated data"

If you have mutations on rows in multiple tables, you must specify spanner.commit_timestamp() (or the client library constant) for the commit timestamp column in each table.

Query a commit timestamp column

The following example queries the commit timestamp column of the table.

C++

voidQueryDataWithTimestamp(google::cloud::spanner::Clientclient){namespacespanner=::google::cloud::spanner;spanner::SqlStatementselect("SELECT SingerId, AlbumId, MarketingBudget, LastUpdateTime"" FROM Albums"" ORDER BY LastUpdateTime DESC");usingRowType=std::tuple<std::int64_t,std::int64_t,absl::optional<std::int64_t>,absl::optional<spanner::Timestamp>>;autorows=client.ExecuteQuery(std::move(select));for(auto&row:spanner::StreamOf<RowType>(rows)){if(!row)throwstd::move(row).status();std::cout << std::get<0>(*row) << " " << std::get<1>(*row);automarketing_budget=std::get<2>(*row);if(!marketing_budget){std::cout << " NULL";}else{std::cout << ' ' << *marketing_budget;}autolast_update_time=std::get<3>(*row);if(!last_update_time){std::cout << " NULL";}else{std::cout << ' ' << *last_update_time;}std::cout << "\n";}}

C#

usingGoogle.Cloud.Spanner.Data;usingSystem;usingSystem.Collections.Generic;usingSystem.Threading.Tasks;publicclassQueryDataWithTimestampColumnAsyncSample{publicclassAlbum{publicintSingerId{get;set;}publicintAlbumId{get;set;}publicDateTime?LastUpdateTime{get;set;}publiclong?MarketingBudget{get;set;}}publicasyncTask<List<Album>>QueryDataWithTimestampColumnAsync(stringprojectId,stringinstanceId,stringdatabaseId){stringconnectionString=$"Data Source=projects/{projectId}/instances/{instanceId}/databases/{databaseId}";usingvarconnection=newSpannerConnection(connectionString);usingvarcmd=connection.CreateSelectCommand("SELECT SingerId, AlbumId, MarketingBudget, LastUpdateTime FROM Albums ORDER BY LastUpdateTime DESC");varalbums=newList<Album>();usingvarreader=awaitcmd.ExecuteReaderAsync();while(awaitreader.ReadAsync()){albums.Add(newAlbum{SingerId=reader.GetFieldValue<int>("SingerId"),AlbumId=reader.GetFieldValue<int>("AlbumId"),LastUpdateTime=reader.IsDBNull(reader.GetOrdinal("LastUpdateTime"))?(DateTime?)null:reader.GetFieldValue<DateTime>("LastUpdateTime"),MarketingBudget=reader.IsDBNull(reader.GetOrdinal("MarketingBudget"))?0:reader.GetFieldValue<long>("MarketingBudget")});}returnalbums;}}

Go

import("context""fmt""io""strconv""cloud.google.com/go/spanner""google.golang.org/api/iterator")funcqueryWithTimestamp(wio.Writer,dbstring)error{ctx:=context.Background()client,err:=spanner.NewClient(ctx,db)iferr!=nil{returnerr}deferclient.Close()stmt:=spanner.Statement{SQL:`SELECT SingerId, AlbumId, MarketingBudget, LastUpdateTime FROM Albums ORDER BY LastUpdateTime DESC`}iter:=client.Single().Query(ctx,stmt)deferiter.Stop()for{row,err:=iter.Next()iferr==iterator.Done{returnnil}iferr!=nil{returnerr}varsingerID,albumIDint64varmarketingBudgetspanner.NullInt64varlastUpdateTimespanner.NullTimeiferr:=row.ColumnByName("SingerId",&singerID);err!=nil{returnerr}iferr:=row.ColumnByName("AlbumId",&albumID);err!=nil{returnerr}iferr:=row.ColumnByName("MarketingBudget",&marketingBudget);err!=nil{returnerr}budget:="NULL"ifmarketingBudget.Valid{budget=strconv.FormatInt(marketingBudget.Int64,10)}iferr:=row.ColumnByName("LastUpdateTime",&lastUpdateTime);err!=nil{returnerr}timestamp:="NULL"iflastUpdateTime.Valid{timestamp=lastUpdateTime.String()}fmt.Fprintf(w,"%d %d %s %s\n",singerID,albumID,budget,timestamp)}}

Java

staticvoidqueryMarketingBudgetWithTimestamp(DatabaseClientdbClient){// Rows without an explicit value for MarketingBudget will have a MarketingBudget equal to// null. A try-with-resource block is used to automatically release resources held by// ResultSet.try(ResultSetresultSet=dbClient.singleUse().executeQuery(Statement.of("SELECT SingerId, AlbumId, MarketingBudget, LastUpdateTime FROM Albums"+" ORDER BY LastUpdateTime DESC"))){while(resultSet.next()){System.out.printf("%d %d %s %s\n",resultSet.getLong("SingerId"),resultSet.getLong("AlbumId"),// We check that the value is non null. ResultSet getters can only be used to retrieve// non null values.resultSet.isNull("MarketingBudget")?"NULL":resultSet.getLong("MarketingBudget"),resultSet.isNull("LastUpdateTime")?"NULL":resultSet.getTimestamp("LastUpdateTime"));}}}

Node.js

// ...// Imports the Google Cloud client libraryconst{Spanner}=require('@google-cloud/spanner');/** * TODO(developer): Uncomment the following lines before running the sample. */// const projectId = 'my-project-id';// const instanceId = 'my-instance';// const databaseId = 'my-database';// Creates a clientconstspanner=newSpanner({projectId:projectId,});// Gets a reference to a Cloud Spanner instance and databaseconstinstance=spanner.instance(instanceId);constdatabase=instance.database(databaseId);constquery={sql:`SELECT SingerId, AlbumId, MarketingBudget, LastUpdateTime FROM Albums ORDER BY LastUpdateTime DESC`,};// Queries rows from the Albums tabletry{const[rows]=awaitdatabase.run(query);rows.forEach(row=>{constjson=row.toJSON();console.log(`SingerId: ${json.SingerId}, AlbumId: ${json.AlbumId}, MarketingBudget: ${json.MarketingBudget?json.MarketingBudget:null}, LastUpdateTime: ${json.LastUpdateTime}`);});}catch(err){console.error('ERROR:',err);}finally{// Close the database when finisheddatabase.close();}

PHP

use Google\Cloud\Spanner\SpannerClient;/** * Queries sample data from a database with a commit timestamp column. * * This sample uses the `MarketingBudget` column. You can add the column * by running the `add_column` sample or by running this DDL statement against * your database: * * ALTER TABLE Albums ADD COLUMN MarketingBudget INT64 * * This sample also uses the 'LastUpdateTime' commit timestamp column. You can * add the column by running the `add_timestamp_column` sample or by running * this DDL statement against your database: * * ALTER TABLE Albums ADD COLUMN LastUpdateTime TIMESTAMP OPTIONS (allow_commit_timestamp=true) * * Example: * ``` * query_data_with_timestamp_column($instanceId, $databaseId); * ``` * * @param string $instanceId The Spanner instance ID. * @param string $databaseId The Spanner database ID. */function query_data_with_timestamp_column(string $instanceId, string $databaseId): void{ $spanner = new SpannerClient(); $instance = $spanner->instance($instanceId); $database = $instance->database($databaseId); $results = $database->execute( 'SELECT SingerId, AlbumId, MarketingBudget, LastUpdateTime ' . ' FROM Albums ORDER BY LastUpdateTime DESC' ); foreach ($results as $row) { if ($row['MarketingBudget'] == null) { $row['MarketingBudget'] = 'NULL'; } if ($row['LastUpdateTime'] == null) { $row['LastUpdateTime'] = 'NULL'; } printf('SingerId: %s, AlbumId: %s, MarketingBudget: %s, LastUpdateTime: %s' . PHP_EOL, $row['SingerId'], $row['AlbumId'], $row['MarketingBudget'], $row['LastUpdateTime']); }}

Python

defquery_data_with_timestamp(instance_id,database_id):"""Queries sample data from the database using SQL. This updates the `LastUpdateTime` column which must be created before running this sample. You can add the column by running the `add_timestamp_column` sample or by running this DDL statement against your database: ALTER TABLE Performances ADD COLUMN LastUpdateTime TIMESTAMP OPTIONS (allow_commit_timestamp=true) """spanner_client=spanner.Client()instance=spanner_client.instance(instance_id)database=instance.database(database_id)withdatabase.snapshot()assnapshot:results=snapshot.execute_sql("SELECT SingerId, AlbumId, MarketingBudget FROM Albums ""ORDER BY LastUpdateTime DESC")forrowinresults:print("SingerId: {}, AlbumId: {}, MarketingBudget: {}".format(*row))

Ruby

# project_id = "Your Google Cloud project ID"# instance_id = "Your Spanner instance ID"# database_id = "Your Spanner database ID"require"google/cloud/spanner"spanner=Google::Cloud::Spanner.newproject:project_idclient=spanner.clientinstance_id,database_idclient.execute("SELECT SingerId, AlbumId, MarketingBudget, LastUpdateTime FROM Albums ORDER BY LastUpdateTime DESC").rows.eachdo|row|puts"#{row[:SingerId]}#{row[:AlbumId]}#{row[:MarketingBudget]}#{row[:LastUpdateTime]}"end

Provide your own value for the commit timestamp column

In your code, you can provide your own value for the commit timestamp column instead of passing spanner.commit_timestamp() (or the available client library constant) as the column value. The value must be a timestamp in the past. This restriction ensures that writing timestamps is an inexpensive and fast operation. One way to confirm that a value is in the past is to compare it to the value returned by the CURRENT_TIMESTAMP SQL function. The server returns a FailedPrecondition error if a future timestamp is specified.

Create a changelog

Suppose that you want to create a changelog of every mutation that happens to a table and then use that changelog for auditing. An example would be a table that stores the history of changes to word processing documents. The commit timestamp makes creating the changelog easier, because the timestamps can enforce ordering of the changelog entries. You could build a changelog that stores the history of changes to a given document using a schema like the following example:

CREATETABLEDocuments(UserIdint8NOTNULL,DocumentIdint8NOTNULL,ContentstextNOTNULL,PRIMARYKEY(UserId,DocumentId));CREATETABLEDocumentHistory(UserIdint8NOTNULL,DocumentIdint8NOTNULL,TsSPANNER.COMMIT_TIMESTAMPNOTNULL,Deltatext,PRIMARYKEY(UserId,DocumentId,Ts))INTERLEAVEINPARENTDocuments;

To create a changelog, insert a new row in DocumentHistory in the same transaction in which you insert or update a row in Document. In the insertion of the new row in DocumentHistory, use the placeholder spanner.commit_timestamp() (or client library constant) to tell Spanner to write the commit timestamp into column Ts. Interleaving the DocumentsHistory table with the Documents table permits data locality and more efficient inserts and updates. However, it also adds the constraint that the parent and child rows must be deleted together. To keep the rows in DocumentHistory after rows in Documents are deleted, don't interleave the tables.

Optimize recent-data queries with commit timestamps

Commit timestamps optimize your Spanner database and can reduce query I/O when retrieving data written after a particular time.

To activate this optimization, a query's WHERE clause must include a comparison between a table's commit timestamp column and a specific time that you provide, with the following attributes:

  • Provide the specific time as a constant expression: a literal, a parameter, or a function whose own arguments evaluate to constants.

  • Compare whether the commit timestamp is more recent than the given time, through either the > or >= operators.

  • Optionally, add further restrictions to the WHERE clause with AND. Extending the clause with OR disqualifies the query from this optimization.

For example, consider the following Performances table, which includes a commit timestamp column:

CREATETABLEPerformances(SingerIdbigintNOTNULL,VenueIdbigintNOTNULL,EventDatetimestampwithtimezoneNOTNULL,Revenuebigint,LastUpdateTimespanner.commit_timestamp,PRIMARYKEY(SingerId,VenueId,EventDate));

This query benefits from the commit-timestamp optimization described earlier, because it has a greater-than-or-equal-to comparison between the table's commit timestamp column and a constant expression—in this case, a literal:

SELECT*FROMPerformancesWHERELastUpdateTime>='2022-01-01';