使用次要索引

次要索引可透過定義您在查詢和掃描作業中使用的替代索引鍵來改善資料存取。全域次要索引 (GSI) 具有分割索引鍵和排序索引鍵，可能與基底資料表上的索引鍵不同。相反地，本機次要索引 (LSI) 會使用主索引的分割區索引鍵。

使用輔助索引註釋註釋數據類

參與次要索引的屬性需要@DynamoDbSecondaryPartitionKey或@DynamoDbSecondarySortKey註釋。

下面的類顯示了兩個索引的註釋。名為的 GSI SubjectLastPostedDateIndex會使用分割索引鍵的Subject屬性，以及排序索LastPostedDateTime引鍵的屬性。名為的 LSI ForumLastPostedDateIndex會使用ForumName做為其分割區索引鍵與LastPostedDateTime排序索引鍵。

請注意，該Subject屬性具有雙重角色。它是主鍵的排序鍵和名為SubjectLastPostedDateIndex的 GSI 的分區鍵。

MessageThread 類別

此MessageThread類別適合做為 Amazon DynamoDB 開發人員指南中「執行緒」表範例表格的資料類別使用。

匯入

import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbBean;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbPartitionKey;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbSecondaryPartitionKey;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbSecondarySortKey;
import software.amazon.awssdk.enhanced.dynamodb.mapper.annotations.DynamoDbSortKey;

import java.util.List;

@DynamoDbBean
public class MessageThread {
    private String ForumName;
    private String Subject;
    private String Message;
    private String LastPostedBy;
    private String LastPostedDateTime;
    private Integer Views;
    private Integer Replies;
    private Integer Answered;
    private List<String> Tags;

    @DynamoDbPartitionKey
    public String getForumName() {
        return ForumName;
    }

    public void setForumName(String forumName) {
        ForumName = forumName;
    }

    // Sort key for primary index and partition key for GSI "SubjectLastPostedDateIndex".
    @DynamoDbSortKey
    @DynamoDbSecondaryPartitionKey(indexNames = "SubjectLastPostedDateIndex")
    public String getSubject() {
        return Subject;
    }

    public void setSubject(String subject) {
        Subject = subject;
    }

    // Sort key for GSI "SubjectLastPostedDateIndex" and sort key for LSI "ForumLastPostedDateIndex".
    @DynamoDbSecondarySortKey(indexNames = {"SubjectLastPostedDateIndex", "ForumLastPostedDateIndex"})
    public String getLastPostedDateTime() {
        return LastPostedDateTime;
    }

    public void setLastPostedDateTime(String lastPostedDateTime) {
        LastPostedDateTime = lastPostedDateTime;
    }
    public String getMessage() {
        return Message;
    }

    public void setMessage(String message) {
        Message = message;
    }

    public String getLastPostedBy() {
        return LastPostedBy;
    }

    public void setLastPostedBy(String lastPostedBy) {
        LastPostedBy = lastPostedBy;
    }

    public Integer getViews() {
        return Views;
    }

    public void setViews(Integer views) {
        Views = views;
    }

    @DynamoDbSecondaryPartitionKey(indexNames = "ForumRepliesIndex")
    public Integer getReplies() {
        return Replies;
    }

    public void setReplies(Integer replies) {
        Replies = replies;
    }

    public Integer getAnswered() {
        return Answered;
    }

    public void setAnswered(Integer answered) {
        Answered = answered;
    }

    public List<String> getTags() {
        return Tags;
    }

    public void setTags(List<String> tags) {
        Tags = tags;
    }

    public MessageThread() {
        this.Answered = 0;
        this.LastPostedBy = "";
        this.ForumName = "";
        this.Message = "";
        this.LastPostedDateTime = "";
        this.Replies = 0;
        this.Views = 0;
        this.Subject = "";
    }

    @Override
    public String toString() {
        return "MessageThread{" +
                "ForumName='" + ForumName + '\'' +
                ", Subject='" + Subject + '\'' +
                ", Message='" + Message + '\'' +
                ", LastPostedBy='" + LastPostedBy + '\'' +
                ", LastPostedDateTime='" + LastPostedDateTime + '\'' +
                ", Views=" + Views +
                ", Replies=" + Replies +
                ", Answered=" + Answered +
                ", Tags=" + Tags +
                '}';
    }
}

顯示較多顯示較少

建立索引

從 SDK for Java 2.20.86 版本開始，該createTable()方法會自動從數據類註釋生成輔助索引。根據預設，基礎資料表中的所有屬性都會複製到索引，而佈建的輸送量值為 20 個讀取容量單位和 20 個寫入容量單位。

但是，如果您使用 2.20.86 之前的 SDK 版本，則需要建立索引與表格，如下列範例所示。這個範例會建立Thread資料表的兩個索引。生成器參數具有配置兩種類型的索引的方法，如註釋行 1 和 2 之後所示。您可以使用索引產生器的indexName()方法，將資料類別註釋中指定的索引名稱與預期的索引類型相關聯。

此代碼配置所有表格屬性，以便在註釋行 3 和 4 之後的兩個索引中結束。有關屬性預測的更多資訊，請參閱 Amazon DynamoDB 開發人員指南。

    public static void createMessageThreadTable(DynamoDbTable<MessageThread> messageThreadDynamoDbTable, DynamoDbClient dynamoDbClient) {
        messageThreadDynamoDbTable.createTable(b -> b
                // 1. Generate the GSI.
                .globalSecondaryIndices(gsi -> gsi.indexName("SubjectLastPostedDateIndex")
                        // 3. Populate the GSI with all attributes.
                        .projection(p -> p
                                .projectionType(ProjectionType.ALL))
                )
                // 2. Generate the LSI.
                .localSecondaryIndices(lsi -> lsi.indexName("ForumLastPostedDateIndex")
                        // 4. Populate the LSI with all attributes.
                        .projection(p -> p
                                .projectionType(ProjectionType.ALL))
                )
        );

使用索引進行查詢

下列範例會查詢本機次要索引ForumLastPostedDateIndex。

在註解第 2 行之後，您可以QueryConditional建立呼叫 DynamoDbIndex.query () 方法時所需的物件。

通過傳遞索引的名稱，您可以在註釋行 3 之後獲得對要查詢的索引的引用。在註釋第 4 行之後，您可以在傳入QueryConditional對象的索引上調用該query()方法。

您也可以將查詢設定為傳回三個屬性值，如註解行 5 之後所示。如果attributesToProject()未呼叫，查詢會傳回所有屬性值。請注意，指定的屬性名稱以小寫字母開頭。這些屬性名稱與表格中使用的屬性名稱相符，而不一定是資料類別的屬性名稱。

在註釋第 6 行之後，遍歷結果並記錄查詢返回的每個項目，並將其存儲在列表中以返回給調用者。

public static List<MessageThread> queryUsingSecondaryIndices(DynamoDbEnhancedClient enhancedClient,
                                                                 String lastPostedDate,
                                                                 DynamoDbTable<MessageThread> threadTable) {
        // 1. Log the parameter value.
        logger.info("lastPostedDate value: {}", lastPostedDate);

        // 2. Create a QueryConditional whose sort key value must be greater than or equal to the parameter value.
        QueryConditional queryConditional = QueryConditional.sortGreaterThanOrEqualTo(qc ->
                qc.partitionValue("Forum02").sortValue(lastPostedDate));

        // 3. Specify the index name to query the DynamoDbIndex instance.
        final DynamoDbIndex<MessageThread> forumLastPostedDateIndex = threadTable.index("ForumLastPostedDateIndex");

        // 4. Perform the query by using the QueryConditional object.
        final SdkIterable<Page<MessageThread>> pagedResult = forumLastPostedDateIndex.query(q -> q
                .queryConditional(queryConditional)
                // 5. Request three attribute in the results.
                .attributesToProject("forumName", "subject", "lastPostedDateTime"));

        List<MessageThread> collectedItems = new ArrayList<>();
        // 6. Iterate through the pages response and sort the items.
        pagedResult.stream().forEach(page -> page.items().stream()
                        .sorted(Comparator.comparing(MessageThread::getLastPostedDateTime))
                        .forEach(mt -> {
                            // 7. Log the returned items and add the collection to return to the caller.
                            logger.info(mt.toString());
                            collectedItems.add(mt);
                        }));
        return collectedItems;
    }

在執行查詢之前，資料庫中存在下列項目。

MessageThread{ForumName='Forum01', Subject='Subject01', Message='Message01', LastPostedBy='', LastPostedDateTime='2023.03.28', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum02', Subject='Subject02', Message='Message02', LastPostedBy='', LastPostedDateTime='2023.03.29', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum02', Subject='Subject04', Message='Message04', LastPostedBy='', LastPostedDateTime='2023.03.31', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum02', Subject='Subject08', Message='Message08', LastPostedBy='', LastPostedDateTime='2023.04.04', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum02', Subject='Subject10', Message='Message10', LastPostedBy='', LastPostedDateTime='2023.04.06', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum03', Subject='Subject03', Message='Message03', LastPostedBy='', LastPostedDateTime='2023.03.30', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum03', Subject='Subject06', Message='Message06', LastPostedBy='', LastPostedDateTime='2023.04.02', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum03', Subject='Subject09', Message='Message09', LastPostedBy='', LastPostedDateTime='2023.04.05', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum05', Subject='Subject05', Message='Message05', LastPostedBy='', LastPostedDateTime='2023.04.01', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum07', Subject='Subject07', Message='Message07', LastPostedBy='', LastPostedDateTime='2023.04.03', Views=0, Replies=0, Answered=0, Tags=null}

第 1 行和第 6 行的記錄陳述式會產生下列主控台輸出。

lastPostedDate value: 2023.03.31
MessageThread{ForumName='Forum02', Subject='Subject04', Message='', LastPostedBy='', LastPostedDateTime='2023.03.31', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum02', Subject='Subject08', Message='', LastPostedBy='', LastPostedDateTime='2023.04.04', Views=0, Replies=0, Answered=0, Tags=null}
MessageThread{ForumName='Forum02', Subject='Subject10', Message='', LastPostedBy='', LastPostedDateTime='2023.04.06', Views=0, Replies=0, Answered=0, Tags=null}

查詢會傳回值為論壇 02 且forumNamelastPostedDateTime值大於或等於 2023.03.31 的項目。雖然message屬性在索引中具有message值，但結果會顯示含有空字串的值。這是因為訊息屬性不會在註解行 5 之後投影。