Firebolt’s cost-based rules rely on a notion of cost based on the estimated number of output rows (that is, the output cardinality) of each sub-plan. Among all alternatives, these rules apply the transformation that results in the sub-plan with the lowest cost. To establish the cost of each sub-plan, the optimizer derives meta-information known at this point in the form of a logical profile in a bottom-up manner.

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Logical profiles

The logical profile of a sub-plan consists of an estimate for the number of produced rows, and optional column-level estimates (such as number of distinct values). In addition to that, the profile has a source that reflects how this information was computed.
  • The default statistics source serves hard-coded values for the number of rows in a table. These values depend only on the table type, and not on the actual data contained in the table.
  • The storage manager statistics source serves row counts based on metadata maintained by the storage manager. These statistics are always up-to-date.
  • The estimated source is assigned to profiles that were computed using Firebolt’s estimation model. These profiles are computed by the optimizer based on the logical profiles of the sub-plan inputs.
You can inspect the logical profiles of a query plan by using the EXPLAIN command with the statistics option. The following code snippet shows the logical profiles of a simple query: 
explain(logical, statistics)
select
  ss_quantity, ss_list_price, ss_net_profit
from
  store_sales
where
  ss_item_sk = 42
order by
  ss_net_profit desc
limit
  10
Here are some key observations:
  • The profile of the StoredTable node has metadata source, reflecting the fact that the row count estimate was obtained the metadata served by the storage manager. The value (2880400) accurately reflects the current number of records in the store_sales table.
  • All other profiles have estimated source.
  • The profile of the Filter node reflects the fact that after applying the ss_item_sk = 42 filter, the number distinct ss_item_sk will be 1.
  • The profile of the Sort node (which also applies the limit 10 clause) reflects the fact that the number of output rows will be 10.
  • The profiles of the two Projection nodes inherit the profiles of their inputs.

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Controlling statistics sources

Firebolt gives you the ability to turn storage manager statistics on and off using the enable_storage_statistics session parameter. Here is an example that uses the same query as above in a session context where enable_storage_statistics is set to false. Observe that the logical profile of the StoredTable node now has source hardcoded, and the estimated number of rows is 100 million.
enable_storage_statistics = false
[0] [Projection] store_sales.ss_quantity, store_sales.ss_list_price, store_sales.ss_net_profit
|   [Logical Profile]: [est. #rows=10, source: estimated]
 \_[1] [Sort] OrderBy: [store_sales.ss_net_profit Descending First] Limit: [10]
   |   [Logical Profile]: [est. #rows=10, source: estimated]
    \_[2] [Projection] store_sales.ss_quantity, store_sales.ss_list_price, store_sales.ss_net_profit
      |   [Logical Profile]: [est. #rows=10000, source: estimated]
       \_[3] [Filter] (store_sales.ss_item_sk = 42)
         |   [Logical Profile]: [est. #rows=10000, column profiles={[store_sales.ss_item_sk: #distinct=1]}, source: estimated]
          \_[4] [StoredTable] Name: "store_sales"
                [Logical Profile]: [est. #rows=1e+08, source: hardcoded]