A pull query is a form of query issued by a client that retrieves a result as of "now", like a query against a traditional RDBS.
As a dual to the push query example, a pull query for a geographic location would ask for the current map coordinates of a particular user. Because it's a pull query, it returns immediately with a finite result and closes its connection. This is ideal for rendering a user interface once, at page load time. It's generally a good fit for any sort of synchronous control flow.
Pull queries enable you to fetch the current state of a materialized view. Because materialized views are incrementally updated as new events arrive, pull queries run with predictably low latency. They're a great match for request/response flows. For asynchronous application flows, see Push Query.
Execute a pull query by sending an HTTP request to the ksqlDB REST API, and the API responds with a single response.
Pull query features and limitations¶
- Pull queries are expressed using a strict subset of ANSI SQL.
- Pull queries are currently available only against materialized aggregate tables, which means tables that are created by using a persistent query with a GROUP BY clause.
- For non-windowed aggregations, pull queries only support looking up events by key.
- WHERE clauses can only have
ROWKEY=x-style bounds for non-windowed tables.
- Windowed tables support bounds on WINDOWSTART using operators
- JOIN, PARTITION BY, GROUP BY and WINDOW clauses aren't supported.
- SELECT statements can contain column arithmetic and function calls.
- The result of a pull query isn't persisted anywhere.
Example pull query¶
The following pull query gets all events for the specified user that have a
timestamp within the specified time window. The WHERE clause must contain a
single value of
ROWKEY to retrieve and may optionally include bounds on
WINDOWSTART if the materialized table is windowed.
SELECT * FROM user_location WHERE ROWKEY = 'user19r7t33' AND '2019-10-02T21:31:16' <= WINDOWSTART AND WINDOWSTART <= '2019-10-03T21:31:16';