Types of Primary and Secondary Index

concept

Creating the right index — with the right keys, in the right order, and using the right expressions — is critical to query performance in any database system. This is true for Couchbase as well. This topic provides an overview of the types of index that you can create using the Index Service, and explains how they help to query for data efficiently and improve query performance.

Examples on this Page

The examples in this topic use the travel-sample dataset which is shipped with Couchbase Server. For instructions on how to install the sample bucket, see Sample Buckets.

To use the examples on this page, you must set the query context to the inventory scope in the travel sample dataset. For more information, see Query Context.

Introduction: Document Keys

Couchbase Server is a distributed database that supports flexible data model using JSON. Each document in a keyspace has a user-generated unique document key and this uniqueness is enforced when inserting data into the keyspace. Consider the following sample document:

SELECT meta().id, travel
FROM airline travel
LIMIT 1;

[
    {
        "id": "airline_10", (1)
        "travel": {
            "callsign": "MILE-AIR",
            "country": "United States",
            "iata": "Q5",
            "icao": "MLA",
            "id": 10,
            "name": "40-Mile Air",
            "type": "airline"
        } (2)
    }
]

1	The document key
2	The document content

Primary Index

The primary index is simply an index on the document key on the entire keyspace.

CREATE PRIMARY INDEX ON airline;

The Couchbase data layer enforces the uniqueness constraint on the document key. The primary index, like every other index, is maintained asynchronously. Use the primary index for full keyspace scans (primary scans) when the query does not have any filters (predicates) or when no other index or access path can be used.

A primary index does not index any transaction records that may be stored in a keyspace. This means that if you are counting the number of documents in a keyspace, you may see slightly different results, depending on whether you are using a primary index or not. Refer to Aggregate Functions and Viewing the Data Insights.

Example 1. Metadata for Primary Index

SELECT * FROM system:indexes WHERE name = '#primary';

Results

[
  {
    "indexes": {
      "bucket_id": "travel-sample",
      "datastore_id": "http://127.0.0.1:8091",
      "id": "c6f4ec5d935e1626",
      "index_key": [],
      "is_primary": true,
      "keyspace_id": "airline",
      "name": "#primary",
      "namespace_id": "default",
      "scope_id": "inventory",
      "state": "online",
      "using": "gsi"
    }
  }
]

The metadata provides additional information about the index, such as where the index resides (datastore_id), its state (state), and the indexing method used (using).

Named Primary Index

You can name the primary index, as seen in the following example.

CREATE PRIMARY INDEX travel_primary ON airline;

The rest of the features of the primary index are the same, except that this index is named. The advantage of naming a primary index is that you can have multiple primary indexes in the system. Duplicate indexes help with high availability and query load distribution between the indexes. This is true for both primary indexes and secondary indexes.

Secondary Index

The secondary index is an index on any key-value or document-key. This index can use any key within the document and the key can be of any type: scalar, object, or array. The query has to use the same type of object for the query engine to use the index.

Example 2. Key is a Simple Scalar Value

CREATE INDEX `idx-name` ON airline(name);

In this keyspace, name is a simple scalar value such as { "name": "Air France" }.

Example 3. Key is an Object Embedded Within the Document

CREATE INDEX travel_geo on landmark(geo);

In this keyspace, geo is an object embedded within the document, such as:

"geo": {
   "alt": 12,
   "lat": 50.962097,
   "lon": 1.954764
}

Example 4. Keys from Nested Objects

CREATE INDEX travel_geo_alt on landmark(geo.alt);

CREATE INDEX travel_geo_lat on landmark(geo.lat);

Example 5. Keys from an Array of Objects

In the route keyspace, schedule is an array of objects with flight details.

"schedule": [
    {
        "day": 0,
        "flight": "AF198",
        "utc": "10:13:00"
    },
    {
        "day": 0,
        "flight": "AF547",
        "utc": "19:14:00"
    },
    {
        "day": 0,
        "flight": "AF943",
        "utc": "01:31:00"
    },
    {
        "day": 1,
        "flight": "AF356",
        "utc": "12:40:00"
    },
    {
        "day": 1,
        "flight": "AF480",
        "utc": "08:58:00"
    },
    {
        "day": 1,
        "flight": "AF250",
        "utc": "12:59:00"
    }
]

This command indexes the complete array and is useful only if you’re looking for the entire array.

CREATE INDEX travel_schedule ON route(schedule);

Composite Secondary Index

It’s common to have queries with multiple filters (predicates). In such cases, you want to use indexes with multiple keys so the indexes can return only the qualified document keys. Additionally, if a query is referencing only the keys in the index, the query engine can simply answer the query from the index scan result without having to fetch from the data nodes. This is commonly used for performance optimization.

CREATE INDEX travel_info ON airline(name, id, icao, iata);

Each of the keys can be a simple scalar field, object, or an array. For the index filtering to be exploited, the filters have to use respective object type in the query filter.

The keys to the secondary indexes can include document keys (meta().id) explicitly if you need to filter on the document keys in the index.

Functional Index

It’s common to have names in the database with a mix of upper and lower cases. When you need to search, say for the city "Villeneuve-sur-lot", you want to search for all uppercase and lowercase possibilities of it. In order to do so, first create an index using an expression or a function as the key. For example:

CREATE INDEX travel_cxname ON airport(LOWER(name));

If you provide the search string in lowercase, the index helps the query engine more efficiently search for already lowercase values in the index.

EXPLAIN SELECT * FROM airport
WHERE LOWER(name) = "villeneuve-sur-lot";

Results

[
  {
    "plan": {
      "#operator": "Sequence",
      "~children": [
        {
          "#operator": "IndexScan3",
          "bucket": "travel-sample",
          "index": "travel_cxname",
          "index_id": "97307509cbce54ca",
          "index_projection": {
            "primary_key": true
          },
          "keyspace": "airport",
          "namespace": "default",
          "scope": "inventory",
          "spans": [
            {
              "exact": true,
              "range": [
                {
                  "high": "\"villeneuve-sur-lot\"",
                  "inclusion": 3,
                  "low": "\"villeneuve-sur-lot\""
                }
              ]
            }
          ],
          "using": "gsi"
        },
        {
          "#operator": "Fetch",
          "bucket": "travel-sample",
          "keyspace": "airport",
          "namespace": "default",
          "scope": "inventory"
        },
        {
          "#operator": "Parallel",
          "~child": {
            "#operator": "Sequence",
            "~children": [
              {
                "#operator": "Filter",
                "condition": "(lower((`airport`.`name`)) = \"villeneuve-sur-lot\")"
              },
              {
                "#operator": "InitialProject",
                "result_terms": [
                  {
                    "expr": "self",
                    "star": true
                  }
                ]
              }
            ]
          }
        }
      ]
    },
    "text": "SELECT * FROM airport WHERE LOWER(name) = \"villeneuve-sur-lot\";"
  }
]

You can also use complex expressions in the functional index. For example:

CREATE INDEX travel_cx1 ON airport
(LOWER(name), ROUND(geo.alt * 0.3048));

Array Index

JSON is hierarchical. At the top level, it can have scalar fields, objects, or arrays. Each object can nest other objects and arrays; each array can have other objects and arrays, and the nesting can continue. Consider the following example array.

"schedule": [
    {
        "day" : 0,
        "special_flights" : [
            {
                "flight" : "AI111",
                "utc" : "1:11:11"
            },
            {
                "flight" : "AI222",
                "utc" : "2:22:22"
            }
        ]
    },
    {
        "day" : 1,
        "flight" : "AF552",
        "utc" : "14:41:00"
    }
]

With a rich structure as seen in the array schedule, here’s how you index a particular array or a field within the sub-object.

CREATE INDEX travel_sched ON route
(DISTINCT ARRAY v.day FOR v IN schedule END);

This index key is an expression on the array to clearly reference only the elements that need to be indexed.

schedule — the array we’re dereferencing into.
v — the variable implicitly declared to reference each element/object within the array schedule.
v.day — the element within each object of the array schedule.

The following query uses the array index created above.

EXPLAIN SELECT * FROM route
WHERE ANY v IN schedule SATISFIES v.day = 2 END;

Results

[
  {
    "plan": {
      "#operator": "Sequence",
      "~children": [
        {
          "#operator": "DistinctScan",
          "scan": {
            "#operator": "IndexScan3",
            "bucket": "travel-sample",
            "index": "travel_sched",
            "index_id": "7cb7b03a5a2a7522",
            "index_projection": {
              "primary_key": true
            },
            "keyspace": "route",
            "namespace": "default",
            "scope": "inventory",
            "spans": [
              {
                "exact": true,
                "range": [
                  {
                    "high": "2",
                    "inclusion": 3,
                    "low": "2"
                  }
                ]
              }
            ],
            "using": "gsi"
          }
        },
        {
          "#operator": "Fetch",
          "bucket": "travel-sample",
          "keyspace": "route",
          "namespace": "default",
          "scope": "inventory"
        },
        {
          "#operator": "Parallel",
          "~child": {
            "#operator": "Sequence",
            "~children": [
              {
                "#operator": "Filter",
                "condition": "any `v` in (`route`.`schedule`) satisfies ((`v`.`day`) = 2) end"
              },
              {
                "#operator": "InitialProject",
                "result_terms": [
                  {
                    "expr": "self",
                    "star": true
                  }
                ]
              }
            ]
          }
        }
      ]
    },
    "text": "SELECT * FROM route\nWHERE ANY v IN schedule SATISFIES v.day = 2 END;"
  }
]

The scan section shows that this query uses the index created above.

Because the key is a generalized expression, it provides the flexibility to apply additional logic and processing on the data before indexing. For example, you can create functional indexing on elements of each array. As you’re referencing individual fields of the object or element within the array, the index creation, size, and search are efficient.

The index travel_sched stores only the distinct values within an array. To store all elements of an array in an index, do not use the DISTINCT modifier to the expression.

CREATE INDEX travel_sched ON route
(ALL ARRAY v.day FOR v IN schedule END);

For further details and examples, refer to Array Indexing.

Partial Index

Unlike relational systems where each type of row is in a distinct table, Couchbase keyspaces can have documents of various types. You can include a distinguishing field in your document to differentiate distinct types.

For example, the landmark keyspace distinguishes types of landmark using the activity field:

SELECT DISTINCT activity FROM landmark;

Result

[
  {
    "activity": "see"
  },
  {
    "activity": "eat"
  },
  {
    "activity": "do"
  },
  {
    "activity": "drink"
  },
  {
    "activity": "buy"
  },
  {
    "activity": "listing"
  }
]

Since the Couchbase data model is JSON and the JSON schema is flexible, an index may not contain entries to documents with absent index keys.

When you want to create an index of restaurants, you can simply add the distinguishing field for the WHERE clause of the index.

CREATE INDEX travel_eat ON landmark(name, id, address)
WHERE activity='eat';

This creates an index only on documents that have activity='eat'. The queries must include the filter activity='eat' in addition to other filters for this index to qualify.

You can use complex predicates in the WHERE clause of the index. Here are some examples where you can use partial indexes:

Partitioning a large index into multiple indexes using the mod function.
Partitioning a large index into multiple indexes and placing each index into distinct indexer nodes.
Partitioning the index based on a list of values. For example, you can have an index for each state.
Simulating index range partitioning via a range filter in the WHERE clause. Note that SQL++ queries use one partitioned index per query block. Use UNION ALL to have a query exploit multiple partitioned indexes in a single query.

Duplicate Index

Duplicate index isn’t really a special type of index, but a feature of Couchbase indexing. You can create duplicate indexes with distinct names.

CREATE INDEX i1 ON airport(LOWER(name), id, icao)
WHERE country = 'France';

CREATE INDEX i2 ON airport(LOWER(name), id, icao)
WHERE country = 'France';

CREATE INDEX i3 ON airport(LOWER(name), id, icao)
WHERE country = 'France';

All three indexes have identical keys and an identical WHERE clause; the only difference is the name of these indexes. You can choose their physical location using the WITH clause of the CREATE INDEX statement.

During query optimization, the query engine chooses one of the index names as seen in the explain plan. During query execution, these indexes are used in a round-robin fashion to distribute the load. Thus providing scale-out, multi-dimensional scaling, performance, and high availability.

Covering Index

Index selection for a query solely depends on the filters in the WHERE clause of your query. After the index selection is made, the query engine analyzes the query to see if it can be answered using only the data in the index. If it does, the query engine skips retrieving the whole document from the data nodes. This is a performance optimization to keep in mind when designing your indexes.

For further details and examples, refer to Covering Indexes.