General rules for simplifying SQL statements

Question

I\'m looking for some \"inference rules\" (similar to set operation rules or logic rules) which I can use to reduce a SQL query in complexity or size. Does there exist some

Answer 1

My approach is to learn relational theory in general and relational algebra in particular. Then learn to spot the constructs used in SQL to implement operators from the relational algebra (e.g. universal quantification a.k.a. division) and calculus (e.g. existential quantification). The gotcha is that SQL has features not found in the relational model e.g. nulls, which are probably best refactored away anyhow. Recommended reading: SQL and Relational Theory: How to Write Accurate SQL Code By C. J. Date.

In this vein, I'm not convinced "the fact that most SUBSELECTs can be rewritten as a JOIN" represents a simplification.

Take this query for example:

SELECT c 
  FROM T1 
 WHERE c NOT IN ( SELECT c FROM T2 );

Rewrite using JOIN

SELECT DISTINCT T1.c 
  FROM T1 NATURAL LEFT OUTER JOIN T2 
 WHERE T2.c IS NULL;

The join is more verbose!

Alternatively, recognize the construct is implementing an antijoin on the projection of c e.g. pseudo algrbra

T1 { c } antijoin T2 { c }

Simplification using relational operators:

SELECT c FROM T1 EXCEPT SELECT c FROM T2;

Answer 2

Here's a few from working with Oracle 8 & 9 (of course, sometimes doing the opposite might make the query simpler or faster):

Parentheses can be removed if they are not used to override operator precedence. A simple example is when all the boolean operators in your where clause are the same: where ((a or b) or c) is equivalent to where a or b or c.

A sub-query can often (if not always) be merged with the main query to simplify it. In my experience, this often improves performance considerably:

select foo.a,
       bar.a
  from foomatic  foo,
       bartastic bar
 where foo.id = bar.id and
       bar.id = (
         select ban.id
           from bantabulous ban
          where ban.bandana = 42
       )
;

is equivalent to

select foo.a,
       bar.a
  from foomatic    foo,
       bartastic   bar,
       bantabulous ban
 where foo.id = bar.id and
       bar.id = ban.id and
       ban.bandana = 42
;

Using ANSI joins separates a lot of "code monkey" logic from the really interesting parts of the where clause: The previous query is equivalent to

select foo.a,
       bar.a
  from foomatic    foo
  join bartastic   bar on bar.id = foo.id
  join bantabulous ban on ban.id = bar.id
 where ban.bandana = 42
;

If you want to check for the existence of a row, don't use count(*), instead use either rownum = 1 or put the query in a where exists clause to fetch only one row instead of all.

Answer 3

• I suppose the obvious one is look for any Cursors that can be replaced with a SQL 'Set' based operation.
• Next on my list, is look for any correlated sub-queries that can be re-written as a un-correlated query
• In long stored procedures, break out separate SQL statements into their own stored procedures. That way they will get there own cached query plan.
• Look for transactions that can have their scope shortened. I regularly find statements inside a transaction that can safely be outside.
• Sub-selects can often be re-written as straight forward joins (modern optimisers are good at spotting simple ones)

As @Quassnoi mentioned, the Optimiser often does a good job. One way to help it is to ensure indexes and statistics are up to date, and that suitable indexes exist for your query workload.

Answer 4

Given the nature of SQL, you absolutely have to be aware of the performance implications of any refactoring. Refactoring SQL Applications is a good resource on refactoring with a heavy emphasis on performance (see Chapter 5).

Answer 5

To state it different: Having a (complex) query with JOINs, SUBSELECTs, UNIONs is it possible (or not) to reduce it to a simpler, equivalent SQL statement, which is producing the same result, by using some transformation rules?

That's exactly what optimizers do for a living (not that I'm saying they always do this well).

Since SQL is a set based language, there are usually more than one way to transform one query to other.

Like this query:

SELECT  *
FROM    mytable
WHERE   col1 > @value1 OR col2 < @value2

can be transformed into this:

SELECT  *
FROM    mytable
WHERE   col1 > @value1
UNION
SELECT  *
FROM    mytable
WHERE   col2 < @value2

or this:

SELECT  mo.*
FROM    (
        SELECT  id
        FROM    mytable
        WHERE   col1 > @value1
        UNION
        SELECT  id
        FROM    mytable
        WHERE   col2 < @value2
        ) mi
JOIN    mytable mo
ON      mo.id = mi.id

, which look uglier but can yield better execution plans.

One of the most common things to do is replacing this query:

SELECT  *
FROM    mytable
WHERE   col IN
        (
        SELECT  othercol
        FROM    othertable
        )

with this one:

SELECT  *
FROM    mytable mo
WHERE   EXISTS
        (
        SELECT  NULL
        FROM    othertable o
        WHERE   o.othercol = mo.col
        )

In some RDBMS's (like PostgreSQL), DISTINCT and GROUP BY use the different execution plans, so sometimes it's better to replace one with the other:

SELECT  mo.grouper,
        (
        SELECT  SUM(col)
        FROM    mytable mi
        WHERE   mi.grouper = mo.grouper
        )
FROM    (
        SELECT  DISTINCT grouper
        FROM    mytable
        ) mo

vs.

SELECT  mo.grouper, SUM(col)
FROM    mytable
GROUP BY
        mo.grouper

In PostgreSQL, DISTINCT sorts and GROUP BY hashes.

MySQL lacks FULL OUTER JOIN, so it can be rewritten as folloing:

SELECT  t1.col1, t2.col2
FROM    table1 t1
LEFT OUTER JOIN
        table2 t2
ON      t1.id = t2.id

vs.

SELECT  t1.col1, t2.col2
FROM    table1 t1
LEFT JOIN
        table2 t2
ON      t1.id = t2.id
UNION ALL
SELECT  NULL, t2.col2
FROM    table1 t1
RIGHT JOIN
        table2 t2
ON      t1.id = t2.id
WHERE   t1.id IS NULL

, but see this article in my blog on how to do this more efficiently in MySQL:

• Emulating FULL OUTER JOIN in MySQL

This hierarchical query in Oracle:

SELECT  DISTINCT(animal_id) AS animal_id
FROM    animal
START WITH
        animal_id = :id
CONNECT BY
        PRIOR animal_id IN (father, mother)
ORDER BY
        animal_id

can be transformed to this:

SELECT  DISTINCT(animal_id) AS animal_id
FROM    (
        SELECT  0 AS gender, animal_id, father AS parent
        FROM    animal
        UNION ALL
        SELECT  1, animal_id, mother
        FROM    animal
        )
START WITH
        animal_id = :id
CONNECT BY
        parent = PRIOR animal_id
ORDER BY
        animal_id

, the latter one being more performant.

See this article in my blog for the execution plan details:

• Genealogy query on both parents

To find all ranges that overlap the given range, you can use the following query:

SELECT  *
FROM    ranges
WHERE   end_date >= @start
        AND start_date <= @end

, but in SQL Server this more complex query yields same results faster:

SELECT  *
FROM    ranges
WHERE   (start_date > @start AND start_date <= @end)
        OR (@start BETWEEN start_date AND end_date)

, and believe it or not, I have an article in my blog on this too:

• Overlapping ranges: SQL Server

SQL Server also lacks an efficient way to do cumulative aggregates, so this query:

SELECT  mi.id, SUM(mo.value) AS running_sum
FROM    mytable mi
JOIN    mytable mo
ON      mo.id <= mi.id
GROUP BY
        mi.id

can be more efficiently rewritten using, Lord help me, cursors (you heard me right: cursors, more efficiently and SQL Server in one sentence).

See this article in my blog on how to do it:

• Flattening timespans: SQL Server

There is a certain kind of query commonly met in financial applications that searches for the effective rate for a currency, like this one in Oracle:

SELECT  TO_CHAR(SUM(xac_amount * rte_rate), 'FM999G999G999G999G999G999D999999')
FROM    t_transaction x
JOIN    t_rate r
ON      (rte_currency, rte_date) IN
        (
        SELECT  xac_currency, MAX(rte_date)
        FROM    t_rate
        WHERE   rte_currency = xac_currency
                AND rte_date <= xac_date
        )

This query can be heavily rewritten to use an equality condition which allows a HASH JOIN instead of NESTED LOOPS:

WITH v_rate AS
        (
        SELECT  cur_id AS eff_currency, dte_date AS eff_date, rte_rate AS eff_rate
        FROM    (
                SELECT  cur_id, dte_date,
                        (
                        SELECT  MAX(rte_date)
                        FROM    t_rate ri
                        WHERE   rte_currency = cur_id
                                AND rte_date <= dte_date
                        ) AS rte_effdate
                FROM    (
                        SELECT  (
                                SELECT  MAX(rte_date)
                                FROM    t_rate
                                ) - level + 1 AS dte_date
                        FROM    dual
                        CONNECT BY
                                level <=
                                (
                                SELECT  MAX(rte_date) - MIN(rte_date)
                                FROM    t_rate
                                )
                        ) v_date,
                        (
                        SELECT  1 AS cur_id
                        FROM    dual
                        UNION ALL
                        SELECT  2 AS cur_id
                        FROM    dual
                        ) v_currency
                ) v_eff
        LEFT JOIN
                t_rate
        ON      rte_currency = cur_id
                AND rte_date = rte_effdate
        )
SELECT  TO_CHAR(SUM(xac_amount * eff_rate), 'FM999G999G999G999G999G999D999999')
FROM    (
        SELECT  xac_currency, TRUNC(xac_date) AS xac_date, SUM(xac_amount) AS xac_amount, COUNT(*) AS cnt
        FROM    t_transaction x
        GROUP BY
                xac_currency, TRUNC(xac_date)
        )
JOIN    v_rate
ON      eff_currency = xac_currency
        AND eff_date = xac_date

Despite being bulky as a hell, the latter query is 6 times faster.

The main idea here is replacing <= with =, which requires building an in-memory calendar table. to JOIN with.

• Converting currencies

Answer 6

I like to replace all sort of subselect by join query.

This one is obvious :

SELECT  *
FROM    mytable mo
WHERE   EXISTS
        (
          SELECT  *
          FROM    othertable o
          WHERE   o.othercol = mo.col
        )

by

SELECT  mo.*
FROM    mytable mo inner join othertable o on o.othercol = mo.col

And this one is under estimate :

SELECT  *
FROM    mytable mo
WHERE   NOT EXISTS
        (
          SELECT  *
          FROM    othertable o
          WHERE   o.othercol = mo.col
        )

by

SELECT  mo.*
FROM    mytable mo left outer join othertable o on o.othercol = mo.col
WHERE   o.othercol is null

It could help the DBMS to choose the good execution plan in a big request.