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Inverted indexes improve your database's performance by helping SQL locate the schemaless data in a JSONB column.
Tip:
For a hands-on demonstration of using an inverted index to improve query performance on a JSONB column, see the JSON tutorial.
How do inverted indexes work?
Standard indexes work well for searches based on prefixes of sorted data. However, schemaless data like JSONB cannot be queried without a full table scan, since it does not adhere to ordinary value prefix comparison operators. JSONB needs to be indexed in a more detailed way than what a standard index provides. This is where inverted indexes prove useful.
Inverted indexes filter on components of tokenizable data. The JSONB data type is built on two structures that can be tokenized:
- Objects - Collections of key-value pairs where each key-value pair is a token.
- Arrays - Ordered lists of values where every value in the array is a token.
For example, take the followingJSONBvalue in columnperson:
{"firstName": "John","lastName": "Smith","age": 25,"address": {"state": "NY","postalCode": "10021"},"cars": ["Subaru","Honda"]}
An inverted index for this object would have an entry per component, mapping it back to the original object:
"firstName": "John""lastName": "Smith""age": 25"address": "state": "NY""address": "postalCode": "10021""cars" : "Subaru""cars" : "Honda"
This lets you to search based on subcomponents.
Creation
You can use inverted indexes to improve the performance of queries using JSONB columns. You can create them:
- At the same time as the table with the
INVERTED INDEXclause ofCREATE TABLE. - For existing tables with
CREATE INVERTED INDEX. - Using the following PostgreSQL-compatible syntax:
> CREATE INDEX <optional name> ON <table> USING GIN (<column>);
Selection
If a query contains a filter against an indexed JSONB column that uses any of the supported operators, the inverted index is added to the set of index candidates.
Because each query can use only a single index, CockroachDB selects the index it calculates will scan the fewest rows (i.e., the fastest). For more detail, check out our blog post Index Selection in CockroachDB.
To override CockroachDB's index selection, you can also force queries to use a specific index (also known as "index hinting").
Storage
CockroachDB stores indexes directly in your key-value store. You can find more information in our blog post Mapping Table Data to Key-Value Storage.
Locking
Tables are not locked during index creation thanks to CockroachDB's schema change procedure.
Performance
Indexes create a trade-off: they greatly improve the speed of queries, but slightly slow down writes (because new values have to be copied and sorted). The first index you create has the largest impact, but additional indexes only introduce marginal overhead.
Comparisons
Currently, inverted indexes only support equality comparisons using the = operator. If you require comparisons using >, <=, etc., you can create an index on a computed column using your JSON payload, and then create a regular index on that. So if you wanted to write a query where the value of "foo" is greater than three, you would:
- Create your table with a computed column:
> CREATE TABLE test (id INT,data JSONB,foo INT AS ((data->>'foo')::INT) STORED);
- Create an index on your computed column:
> CREATE INDEX test_idx ON test (foo);
- Execute your query with your comparison:
> SELECT * FROM test where foo > 3;
Example
In this example, let's create a table with a JSONB column and an inverted index:
> CREATE TABLE users (profile_id UUID PRIMARY KEY DEFAULT gen_random_uuid(),last_updated TIMESTAMP DEFAULT now(),user_profile JSONB,INVERTED INDEX user_details (user_profile));
Then, insert a few rows a data:
> INSERT INTO users (user_profile) VALUES('{"first_name": "Lola", "last_name": "Dog", "location": "NYC", "online" : true, "friends" : 547}'),('{"first_name": "Ernie", "status": "Looking for treats", "location" : "Brooklyn"}'),('{"first_name": "Carl", "last_name": "Kimball", "location": "NYC", "breed": "Boston Terrier"}');
> SELECT *, jsonb_pretty(user_profile) FROM users;
+--------------------------------------+----------------------------------+--------------------------------------------------------------------------+------------------------------------+| profile_id | last_updated | user_profile | jsonb_pretty |+--------------------------------------+----------------------------------+--------------------------------------------------------------------------+------------------------------------+| 81330a51-80b2-44aa-b793-1b8d84ba69c9 | 2018-03-13 18:26:24.521541+00:00 | {"breed": "Boston Terrier", "first_name": "Carl", "last_name": | { || | | "Kimball", "location": "NYC"} | || | | | "breed": "Boston Terrier", || | | | "first_name": "Carl", || | | | "last_name": "Kimball", || | | | "location": "NYC" || | | | } || 81c87adc-a49c-4bed-a59c-3ac417756d09 | 2018-03-13 18:26:24.521541+00:00 | {"first_name": "Ernie", "location": "Brooklyn", "status": "Looking for | { || | | treats"} | || | | | "first_name": "Ernie", || | | | "location": "Brooklyn", || | | | "status": "Looking for treats" || | | | } || ec0a4942-b0aa-4a04-80ae-591b3f57721e | 2018-03-13 18:26:24.521541+00:00 | {"first_name": "Lola", "friends": 547, "last_name": "Dog", "location": | { || | | "NYC", "online": true} | || | | | "first_name": "Lola", || | | | "friends": 547, || | | | "last_name": "Dog", || | | | "location": "NYC", || | | | "online": true || | | | } |+--------------------------------------+----------------------------------+--------------------------------------------------------------------------+------------------------------------+
Now, run a query that filters on the JSONB column:
> SELECT * FROM users where user_profile @> '{"location":"NYC"}';
+--------------------------------------+----------------------------------+--------------------------------------------------------------------------+| profile_id | last_updated | user_profile |+--------------------------------------+----------------------------------+--------------------------------------------------------------------------+| 81330a51-80b2-44aa-b793-1b8d84ba69c9 | 2018-03-13 18:26:24.521541+00:00 | {"breed": "Boston Terrier", "first_name": "Carl", "last_name": || | | "Kimball", "location": "NYC"} || ec0a4942-b0aa-4a04-80ae-591b3f57721e | 2018-03-13 18:26:24.521541+00:00 | {"first_name": "Lola", "friends": 547, "last_name": "Dog", "location": || | | "NYC", "online": true} |+--------------------------------------+----------------------------------+--------------------------------------------------------------------------+(2 rows)
