Question

Problem

72 child tables, each having a year index and a station index, are defined as follows:

CREATE TABLE climate.measurement_12_013
(
-- Inherited from table climate.measurement_12_013:  id bigint NOT NULL DEFAULT nextval('climate.measurement_id_seq'::regclass),
-- Inherited from table climate.measurement_12_013:  station_id integer NOT NULL,
-- Inherited from table climate.measurement_12_013:  taken date NOT NULL,
-- Inherited from table climate.measurement_12_013:  amount numeric(8,2) NOT NULL,
-- Inherited from table climate.measurement_12_013:  category_id smallint NOT NULL,
-- Inherited from table climate.measurement_12_013:  flag character varying(1) NOT NULL DEFAULT ' '::character varying,
  CONSTRAINT measurement_12_013_category_id_check CHECK (category_id = 7),
  CONSTRAINT measurement_12_013_taken_check CHECK (date_part('month'::text, taken)::integer = 12)
)
INHERITS (climate.measurement)

CREATE INDEX measurement_12_013_s_idx
  ON climate.measurement_12_013
  USING btree
  (station_id);
CREATE INDEX measurement_12_013_y_idx
  ON climate.measurement_12_013
  USING btree
  (date_part('year'::text, taken));

(Foreign key constraints to be added later.)

The following query runs abysmally slow due to a full table scan:

SELECT
  count(1) AS measurements,
  avg(m.amount) AS amount
FROM
  climate.measurement m
WHERE
  m.station_id IN (
    SELECT
      s.id
    FROM
      climate.station s,
      climate.city c
    WHERE
        /* For one city... */
        c.id = 5182 AND

        /* Where stations are within an elevation range... */
        s.elevation BETWEEN 0 AND 3000 AND

        /* and within a specific radius... */
        6371.009 * SQRT( 
          POW(RADIANS(c.latitude_decimal - s.latitude_decimal), 2) +
            (COS(RADIANS(c.latitude_decimal + s.latitude_decimal) / 2) *
              POW(RADIANS(c.longitude_decimal - s.longitude_decimal), 2))
        ) <= 50
    ) AND

  /* Data before 1900 is shaky; insufficient after 2009. */
  extract( YEAR FROM m.taken ) BETWEEN 1900 AND 2009 AND

  /* Whittled down by category... */
  m.category_id = 1 AND

  /* Between the selected days and years... */
  m.taken BETWEEN
   /* Start date. */
   (extract( YEAR FROM m.taken )||'-01-01')::date AND
    /* End date. Calculated by checking to see if the end date wraps
       into the next year. If it does, then add 1 to the current year.
    */
    (cast(extract( YEAR FROM m.taken ) + greatest( -1 *
      sign(
        (extract( YEAR FROM m.taken )||'-12-31')::date -
        (extract( YEAR FROM m.taken )||'-01-01')::date ), 0
    ) AS text)||'-12-31')::date
GROUP BY
  extract( YEAR FROM m.taken )

The sluggishness comes from this part of the query:

  m.taken BETWEEN
    /* Start date. */
  (extract( YEAR FROM m.taken )||'-01-01')::date AND
    /* End date. Calculated by checking to see if the end date wraps
      into the next year. If it does, then add 1 to the current year.
    */
    (cast(extract( YEAR FROM m.taken ) + greatest( -1 *
      sign(
        (extract( YEAR FROM m.taken )||'-12-31')::date -
        (extract( YEAR FROM m.taken )||'-01-01')::date ), 0
    ) AS text)||'-12-31')::date

This part of the query matches a selection of days. For example, if the user wants to look at data between June 1st and July 1st over all the years for which there is data, the above clause matches against just those days. If the use wants to look at data between December 22nd and March 22nd, again for all the years for which there is data, the above clause calculates that March 22nd is in the following year of December 22nd, and so matches the date accordingly:

Currently the dates are fixed as Jan 1 to Dec 31, but will be parameterized, as shown above.

The HashAggregate from the plan shows a cost of 10006220141.11, which is, I suspect, on the astronomically huge side.

There is a full table scan on the measurement table (itself having neither data nor indexes) being performed. The table aggregates 273 million rows from its child tables.

Question

What is the proper way to index the dates to avoid full table scans?

Options I have considered:

• GIN
• GiST
• Rewrite the WHERE clause
• Separate year_taken, month_taken, and day_taken columns to the tables

What are your thoughts?

Thank you!

Answer 1

Your problem is that you have a where clause depending on a calculation of the date. There is no way the database can use an index if it needs to fetch every row and do a calculation on it before knowing if the date will match.

Unless you rewrite it to be in the form where the database has a fixed range to check which does not depend on the data it is to retrieve you will always have to scan the table.