[PHOENIX-4550] Declare maximum columns to ensure storage is dense when table has many views

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Assignee: Unassigned
• Reporter: James R. Taylor
• Created: 23/Jan/18 03:14
• Updated: 19/Jul/18 00:13

Description

By declaring the max number of columns on a base table, we can optimize the storage for SINGLE_CELL_ARRAY_WITH_OFFSETS by not storing null values for the columns preceding the initial column of a view. This will make a huge difference in storage when you have a base table with many views. For example:

-- Declare that the base table will have no more than 10 columns
CREATE IMMUTABLE TABLE base (k1 VARCHAR, prefix CHAR(3) v1 DATE,
    CONSTRAINT pk PRIMARY KEY (k1, prefix))
    MULTI_TENANT = true,
    MAX_COLUMNS = 10;
CREATE VIEW v1(k2 VARCHAR PRIMARY KEY, v2 VARCHAR, v3 VARCHAR)
    AS SELECT * FROM base WHERE prefix = 'A00';
CREATE VIEW v2(k2 VARCHAR PRIMARY KEY, v2 VARCHAR, v3 VARCHAR);
    AS SELECT * FROM base WHERE prefix = 'A10';
...

As the number of views grow, the difference between the base table column encoding (column #1) and the starting column number of the view (since the starting offset is determined by an incrementing value on the base table) will increase. This bloats the storage as we need to store null values for column encodings between the base table column and the starting column of the view.

Instead, we'll pass through the MAX_COLUMNS value for queries and anything column encoding less than this we know it'll be at the start. Anything greater and we'll start the search from <column encoding> - <minimum view column encoding>.

The downside of this approach is if you run out of columns in the base table, you're stuck. A more flexible, but more difficult approach is outlined in PHOENIX-4596.