PostgreSQL

07 Feb 2023 - New York

Comparing joins and subqueries

• In SELECT and WHERE clauses: Equivalent once the query optimizer gets done with them
• In FROM clause: JOIN recommended over subquerying

Comparing transient tables and queries

• CTEs: auxiliary statement for use in a larger query, exist for a single query
• Temporary tables: Dropped at the end of a session
• Views: A stored query
• Materialized Views: Stored results of a query view, can be configured to refresh periodically

Analyzing query optimizer

• EXPLAIN ANALYZE: Executes the query being EXPLAIN-ed, providing actual time instead of cost estimates. Careful with mutating queries.

Table partitioning

Partitioning: splitting what is logically one large table into smaller physical pieces.

• Range Partitioning
• List Partitioning
• Hash Partitioning

Window functions

SELECT year, ROW_NUMBER() OVER (ORDER BY year DESC) AS seq_num
FROM -- ...
-- year | seq_num
-- 1999 | 1
-- 1999 | 2
-- 1998 | 3
-- 1997 | 4

SELECT year, ROW_NUMBER() OVER (PARTITION BY year DESC) AS seq_num
FROM -- ...
-- year | seq_num
-- 1999 | 1
-- 1999 | 2
-- 1998 | 1
-- 1997 | 1

SELECT gender, year, champion,
    LAG(champion) OVER (PARTITION BY gender ORDER BY year DESC)
    AS last_champion
FROM -- ...
-- gender | year | champion | last_champion
-- male   | 1990 |  CZE     |  null
-- male   | 1990 |  NOR     |  CZE
-- female | 1990 |  NOR     |  null
-- female | 1990 |  GYE     |  NOR

SELECT SUM(medals) OVER (PARTITION BY country)
SELECT COUNT(medals) OVER (PARTITION BY country)
SELECT AVG(medals) OVER (PARTITION BY country)

Fetching with offsets and frames

• LAG(column, n), LEAD(column, n)
• FIRST_VALUE(column), LAST_VALUE(column)
• RANGE BETWEEN [start] AND [finish]
  • RANGE ignores duplicates in the ORDER BY subclause of OVER
• ROWS BETWEEN [start] AND [finish]
  • n PRECEDING, CURRENT ROW, n FOLLOWING
  • UNBOUNDED PRECEDING
  • UNBOUNDED FOLLOWING

Example

SELECT
    year,
    medals,
    MAX(medals) OVER (
        ORDER BY year ASC
    ) AS max_medals,
    MAX(medals) OVER (
        ORDER BY year ASC
        ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
    ) AS max_medals_thus_far

General SQL heuristics

Indexes

Apply to:

• Large tables
• Frequently used filtering conditions
• Primary key, foreign keys

Don’t apply to:

• Small tables
• Columns with a high number of nulls
• Frequently updated tables

Query Lifecycle

1. Parser
2. Planner & Optimizer
3. Executor

Query Planning Order of Operations