Build SQL for materialized cubes

[shangyian]

Summary

This PR adds functionality to build queries for materialized cube nodes. This was also in the presentation, but I've gone through this a second time to handle various edge cases.

The main changes include:

• When a cube is created, we store some additional info needed for building materialized cube queries into the default "materialization" config, namely a combiner expression that combines the measures back into the metric.
• When building SQL for multiple metrics, we add a check to see if those metrics + dimensions can be found in an existing materialized cube
• If there is a materialized cube that encompasses the metrics + dimensions, we build SQL that queries the cube directly by combining the measures back into the selected metrics
• Refactor so that we have a single function build_sql_for_multiple_metrics that generates SQL for both the /sql and /data endpoints

Example

Let's say we have a cube named default.repairs_cube that contains the following metrics and dimensions:

• default.discounted_orders_rate (metric)
• default.num_repair_orders (metric)
• default.avg_repair_price (metric)
• default.hard_hat.country (dim)
• default.hard_hat.city (dim)

After we post an availability state for this cube, whenever we query metrics or dimensions that are in the cube, we'll generate SQL that queries the materialized cube directly. For example, if we ask for default.discounted_orders_rate, default.num_repair_orders and default.avg_repair_price grouped by default.hard_hat.country, we'll get the following query:

SELECT
  sum(discount_sum) / count(placeholder_count) default_DOT_discounted_orders_rate,
  count(repair_order_id_count) default_DOT_num_repair_orders,
  sum(price_sum) / count(price_count) default_DOT_avg_repair_price,
  country
FROM repairs_cube
GROUP BY
  country

Test Plan

• PR has an associated issue: Cubes --> Decomposing Metrics #470
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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[agorajek]

This is good. Two small comments in line.

[agorajek]

Not necessary, None is the default return value.

[agorajek]

nit: I wonder if it makes sense to add this condition to the find_existing_cube() method itself... perhaps not, but then I feel like we could add a wrapper on that one called find_materialized_cube() and have this condition in there for reuse.

[shangyian]

Agreed, I think it would be a good idea to add an option to look for materialized cubes. I added a parameter to find_existing_cube called materialized so that we can filter for materialized cubes if desired.