Using Persisted Query Results¶

Retrieval Optimization¶

If a user repeats a query that has already been run, and the data in the table(s) hasn’t changed since the last time that the query was run, then the result of the query is the same. Instead of running the query again, Snowflake simply returns the same result that it returned previously. This can substantially reduce query time because Snowflake bypasses query execution and, instead, retrieves the result directly from the cache.

Typically, query results are reused if all of the following conditions are met:

• The new query matches the previously executed query exactly. Any difference in syntax, including lowercase versus uppercase, or the use of table aliases, will inhibit 100% cache reuse. For example, consider the following queries, run in succession:

  SELECT DISTINCT(severity) FROM weather_events;
  SELECT DISTINCT(severity) FROM weather_events;
  SELECT DISTINCT(severity) FROM weather_events we;
  select distinct(severity) from weather_events;
  

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  The first query will populate the cache, and the identical second query will benefit from 100% cache reuse. However, the third and fourth queries will not trigger cache reuse, simply because the third query introduces a table alias and the fourth query uses lowercase keywords.

• The query does not include non-reusable functions, which return different results for successive runs of the same query. UUID_STRING, RANDOM, and RANDSTR are good examples of non-reusable functions.

• The query does not include external functions.

• The query does not select from hybrid tables.

• The table data contributing to the query result has not changed.

• The persisted result for the previous query is still available.

• The role accessing the cached results has the required privileges.

  • If the query was a SELECT query, the role executing the query must have the necessary access privileges for all the tables used in the cached query.

  • If the query was a SHOW query, the role executing the query must match the role that generated the cached results.

• Any configuration options that affect how the result was produced have not changed.

• The table’s micro-partitions have not changed (e.g. been reclustered or consolidated) due to changes to other data in the table.

By default, result reuse is enabled, but can be overridden at the account, user, and session level using the USE_CACHED_RESULT session parameter.

Post-processing Query Results¶

In some cases, you might want to perform further processing on the result of a query that you’ve already run. For example:

• You are developing a complex query step-by-step and you want to add a new layer on top of the previous query and run the new query without recalculating the partial results from scratch.

• The previous query was a SHOW <objects>, DESCRIBE <object>, or CALL statement, which returns results in a form that are not easy to reuse.

  For example, you can’t call a stored procedure inside a more complex SQL statement the way you can call a function inside a SQL statement, so the only way to process the output of the stored procedure is to post-process the stored query results.

Post-processing can be performed using the RESULT_SCAN table function. The function returns the results of the previous query as a “table” and a new query can then be run on the tabular data.