AWS DynamoDB Secondary Indexes

• DynamoDB provides fast access to items in a table by specifying primary key values
• DynamoDB Secondary indexes on a table allow efficient access to data with attributes other than the primary key.
• DynamoDB Secondary indexes
  • is a data structure that contains a subset of attributes from a table.
  • is associated with exactly one table, from which it obtains its data.
  • requires an alternate key for the index partition key and sort key.
  • additionally can define projected attributes that are copied from the base table into the index along with the primary key attributes.
  • is automatically maintained by DynamoDB.
  • indexes on that table are also updated for any addition, modification, or deletion of items in the base table.
  • helps reduce the size of the data as compared to the main table, depending upon the project attributes, and hence helps improve provisioned throughput performance
  • are automatically maintained as sparse objects. Items will only appear in an index if they exist in the table on which the index is defined, making queries an index very efficient
  • use the same table class and capacity mode (provisioned or on-demand) as the base table they are associated with.
• DynamoDB Secondary indexes support two types
  • Global secondary index – an index with a partition key and a sort key that can be different from those on the base table.
  • Local secondary index – an index that has the same partition key as the base table, but a different sort key.
• DynamoDB supports up to 20 global secondary indexes (default quota, can request increase) and up to 5 local secondary indexes per table.

Global Secondary Indexes – GSI

• DynamoDB creates and maintains indexes for the primary key attributes for efficient access to data in the table, which allows applications to quickly retrieve data by specifying primary key values.
• Global Secondary Indexes – GSI are indexes that contain partition or composite partition-and-sort keys that can be different from the keys in the table on which the index is based.
• Global secondary index is considered “global” because queries on the index can span all items in a table, across all partitions.
• Multiple secondary indexes can be created on a table, and queries issued against these indexes.
• Applications benefit from having one or more secondary keys available to allow efficient access to data with attributes other than the primary key.
• GSIs support non-unique attributes, which increases query flexibility by enabling queries against any non-key attribute in the table
• GSIs support eventual consistency only. DynamoDB automatically handles item additions, updates, and deletes in a GSI when corresponding changes are made to the table asynchronously. Strongly consistent reads are NOT supported on GSIs.
• Data in a secondary index consists of GSI alternate key, primary key and attributes that are projected, or copied, from the table into the index.
• Attributes that are part of an item in a table, but not part of the GSI key, the primary key of the table, or projected attributes are not returned on querying the GSI index.
• GSIs can be created at the same time as the table, or added to an existing table. GSIs can also be deleted from an existing table. However, you cannot modify an existing GSI — you must delete and recreate it.
• GSIs inherit the read/write capacity mode from the base table.
  • Provisioned Mode: GSIs manage throughput independently of the table they are based on. The provisioned throughput for the table and each associated GSI needs to be specified at the creation time.
    • Read provisioned throughput
      • provides one Read Capacity Unit with two eventually consistent reads per second for items < 4KB in size.
    • Write provisioned throughput
      • consumes 1 write capacity unit if,
        • a new item is inserted into the table that defines an indexed attribute
        • existing item is deleted from the table
        • existing items are updated for projected attributes
      • consumes 2 write capacity units if
        • existing item is updated for key attributes, which results in deletion and addition of the new item into the index
  • On-Demand Mode: GSIs automatically scale with the table’s traffic. Configurable maximum throughput can optionally limit on-demand capacity for both tables and their associated secondary indexes.
• Throttling on a GSI affects the base table depending on whether the throttling is for read or write activity:
  • When a GSI has insufficient read capacity, the base table isn’t affected.
  • When a GSI has insufficient write capacity, write operations won’t succeed on the base table or any of its GSIs (back-pressure).
• GSIs use the same table class as the base table (DynamoDB Standard or DynamoDB Standard-IA). When the table class is updated, all associated GSIs are updated as well.

Multi-Attribute Composite Keys (November 2025)

• GSIs now support multi-attribute composite keys, allowing partition keys and sort keys to be composed of multiple attributes.
• Partition key can be composed of up to 4 attributes and sort key can be composed of up to 4 attributes, for a total of up to 8 attributes per key schema.
• Eliminates the need to manually concatenate values into synthetic keys (e.g., no more “TOURNAMENT#WINTER2024#REGION#NA-EAST” patterns).
• Multi-attribute partition keys improve data distribution and uniqueness.
• Multi-attribute sort keys enable flexible querying by letting you specify conditions on sort key attributes from left to right (hierarchical querying).
• When querying, all partition key attributes must be specified using equality conditions. Sort key attributes can be queried left-to-right; inequality conditions must be the last condition.
• Each attribute in a multi-attribute key can have its own data type: String (S), Number (N), or Binary (B).
• Multi-attribute keys work particularly well when creating GSIs on existing tables — no need to backfill synthetic keys across data.
• Available at no additional charge in all AWS Regions where DynamoDB is available.
• Note: The base table primary key still uses the traditional structure of a single partition key + optional single sort key. Multi-attribute keys are only for GSIs.

Warm Throughput (November 2024)

• DynamoDB introduced warm throughput for tables and indexes, providing visibility into read and write operations a table or GSI can immediately support.
• Warm throughput values grow automatically as usage increases.
• Tables and GSIs can be pre-warmed by proactively setting higher warm throughput values to prepare for anticipated traffic spikes.
• Warm throughput values are available for all provisioned and on-demand tables and indexes at no cost.
• Pre-warming incurs a charge (see DynamoDB Pricing).
• Default warm throughput for a GSI is 12,000 read units and 4,000 write units.

Local Secondary Indexes (LSI)

• Local secondary indexes are indexes that have the same partition key as the table, but a different sort key.
• Local secondary index is “local” cause every partition of a local secondary index is scoped to a table partition that has the same partition key.
• LSI allows search using a secondary index in place of the sort key, thus expanding the number of attributes that can be used for queries that can be conducted efficiently
• LSI is updated automatically when the primary index is updated and reads support strong, eventual, and transactional consistency options.
• LSIs can only be queried via the Query API
• LSIs cannot be added to existing tables — they can only be created at the same time as the table
• LSIs cannot be modified once created
• LSIs cannot be removed from a table once they are created
• For tables with local secondary indexes, there is a 10 GB size limit per partition key value (item collection size limit). This includes all items in the base table and all items in the LSIs that have the same partition key value.
• LSI consumes provisioned throughput capacity as part of the table with which it is associated
  • Read Provisioned throughput
    • if data read is indexed and projected attributes
      • provides one Read Capacity Unit with one strongly consistent read (or two eventually consistent reads) per second for items < 4KB
      • data size includes the index and projected attributes only
    • if data read is indexed and a non-projected attribute
      • consumes double the read capacity, with one to read from the index and one to read (fetch) from the table with the entire data and not just the non-projected attribute
  • Write provisioned throughput
    • consumes 1 write capacity unit if,
      • a new item is inserted into the table
      • existing item is deleted from the table
      • existing items are updated for projected attributes
    • consumes 2 write capacity units if
      • existing item is updated for key attributes, which results in deletion and addition of the new item into the index

Global Secondary Index vs Local Secondary Index

AWS Certification Exam Practice Questions

• Questions are collected from Internet and the answers are marked as per my knowledge and understanding (which might differ with yours).
• AWS services are updated everyday and both the answers and questions might be outdated soon, so research accordingly.
• AWS exam questions are not updated to keep up the pace with AWS updates, so even if the underlying feature has changed the question might not be updated
• Open to further feedback, discussion and correction.

1. In DynamoDB, a secondary index is a data structure that contains a subset of attributes from a table, along with an alternate key to support ____ operations.
   1. None of the above
   2. Both
   3. Query
   4. Scan
2. In regard to DynamoDB, what is the Global secondary index?
   1. An index with a partition and sort key that can be different from those on the table
   2. An index that has the same sort key as the table, but a different partition key
   3. An index that has the same partition key and sort key as the table
   4. An index that has the same partition key as the table, but a different sort key
3. In regard to DynamoDB, can I modify the index once it is created?
   1. Yes, if it is a primary hash key index
   2. Yes, if it is a Global secondary index (GSIs can be added or deleted on existing tables, but cannot be modified in place — you must delete and recreate)
   3. No
   4. Yes, if it is a local secondary index
4. When thinking of DynamoDB, what is true of Global Secondary Key properties?
   1. Both the partition key and sort key can be different from the table.
   2. Only the partition key can be different from the table.
   3. Either the partition key or the sort key can be different from the table, but not both.
   4. Only the sort key can be different from the table.
5. A team needs to query a DynamoDB table using an attribute that is not part of the primary key. They need strongly consistent reads. Which type of secondary index should they use?
   1. Global Secondary Index with ConsistentRead set to true
   2. Local Secondary Index
   3. Either GSI or LSI supports strongly consistent reads
   4. Neither GSI nor LSI supports strongly consistent reads
6. A DynamoDB table uses on-demand capacity mode. Which of the following statements about secondary indexes on this table is correct?
   1. Secondary indexes must use provisioned capacity mode
   2. GSIs can use a different capacity mode than the base table
   3. Secondary indexes inherit the capacity mode from the base table
   4. Only LSIs can use on-demand capacity mode
7. With the multi-attribute composite key feature for DynamoDB GSIs, what is the maximum number of attributes that can compose a GSI key schema?
   1. 2 (one partition key, one sort key)
   2. 4 (two partition keys, two sort keys)
   3. 8 (up to four partition key attributes, up to four sort key attributes)
   4. 16 (up to eight partition keys, up to eight sort keys)
8. What happens when a Global Secondary Index has insufficient write capacity?
   1. Only reads on the GSI are throttled
   2. The GSI becomes temporarily unavailable
   3. Write operations on the base table and all its GSIs are throttled (back-pressure)
   4. DynamoDB automatically increases the GSI write capacity

References

• AWS DynamoDB Developer Guide – Secondary Indexes
• Using Global Secondary Indexes in DynamoDB
• Local Secondary Indexes in DynamoDB
• Managing Global Secondary Indexes in DynamoDB
• General Guidelines for Secondary Indexes in DynamoDB
• DynamoDB Multi-Attribute Composite Keys for GSIs (Nov 2025)
• Understanding DynamoDB Warm Throughput