- About Optimizer Hints
- August 12–22: Online Training (EU shift)
- Clustered Indexes Dirty Secret
- FOSDEM Impressions
- June 8–18: Online Training (US shift)
- MySQL is to SQL like ??? to NoSQL
- Non-monetary micro sponsoring
- November 11-12 in Frankfurt am Main
- Online-Training in July and August
- Oracle + PostgreSQL
- Oracle Bootcamp
- Party time
- PostgreSQL Performance Event
- Quiz Results
- SQL Server Performance Kurs in Stuttgart
- SQL Server performance training in London
- Shipping Terms
- Springtime for SQL
- The two top performance problems caused by ORM tools
- Top Tweets January 2013
- Training Survey
- Training and Conference Dates
- Use The Index, Luke
Most obfuscations involve
DATE types. The Oracle database is particularly vulnerable in this respect because it has only one
DATE type that always includes a time component as well.
It has become common practice to use the
TRUNC function to remove the time component. In truth, it does not remove the time but instead sets it to midnight because the Oracle database has no pure
DATE type. To disregard the time component for a search you can use the
TRUNC function on both sides of the comparison—e.g., to search for yesterday’s sales:
SELECT ... FROM sales WHERE TRUNC(sale_date) = TRUNC(sysdate - INTERVAL '1' DAY)
It is a perfectly valid and correct statement but it cannot properly make use of an index on
SALE_DATE. It is as explained in “Case-Insensitive Search Using
TRUNC(sale_date) is something entirely different from
SALE_DATE—functions are black boxes to the database.
There is a rather simple solution for this problem: a function-based index.
CREATE INDEX index_name ON table_name (TRUNC(sale_date))
But then you must always use
in the where clause. If you use it inconsistently—sometimes with, sometimes without
TRUNC—then you need two indexes!
The problem also occurs with databases that have a pure date type if you search for a longer period as shown in the following MySQL query:
SELECT ... FROM sales WHERE DATE_FORMAT(sale_date, "%Y-%M") = DATE_FORMAT(now() , "%Y-%M')
The query uses a date format that only contains year and month: again, this is an absolutely correct query that has the same problem as before. However the solution from above does not apply here because MySQL has no function-based indexes.
The alternative is to use an explicit range condition. This is a generic solution that works for all databases:
SELECT ... FROM sales WHERE sale_date BETWEEN quarter_begin(?) AND quarter_end(?)
If you have done your homework, you probably recognize the pattern from the exercise about all employees who are 42 years old.
A straight index on
SALE_DATE is enough to optimize this query. The functions
QUARTER_END compute the boundary dates. The calculation can become a little complex because the between operator always includes the boundary values. The
QUARTER_END function must therefore return a time stamp just before the first day of the next quarter if the
SALE_DATE has a time component. This logic can be hidden in the function.
The following examples show implementations of the functions
QUARTER_END for various databases.
CREATE FUNCTION quarter_begin(dt TIMESTAMP) RETURNS TIMESTAMP RETURN TRUNC(dt, 'Q'); CREATE FUNCTION quarter_end(dt TIMESTAMP) RETURNS TIMESTAMP RETURN TRUNC(dt, 'Q') + 3 MONTHS - 1 SECOND;
CREATE FUNCTION quarter_begin(dt DATETIME) RETURNS DATETIME DETERMINISTIC RETURN CONVERT ( CONCAT ( CONVERT(YEAR(dt),CHAR(4)) , '-' , CONVERT(QUARTER(dt)*3-2,CHAR(2)) , '-01' ) , datetime ); CREATE FUNCTION quarter_end(dt DATETIME) RETURNS DATETIME DETERMINISTIC RETURN DATE_ADD ( DATE_ADD ( quarter_begin(dt), INTERVAL 3 MONTH ) , INTERVAL -1 MICROSECOND);
CREATE FUNCTION quarter_begin(dt IN DATE) RETURN DATE AS BEGIN RETURN TRUNC(dt, 'Q'); END; / CREATE FUNCTION quarter_end(dt IN DATE) RETURN DATE AS BEGIN -- the Oracle DATE type has seconds resolution -- subtract one second from the first -- day of the following quarter RETURN TRUNC(ADD_MONTHS(dt, +3), 'Q') - (1/(24*60*60)); END; /
CREATE FUNCTION quarter_begin(dt timestamp with time zone) RETURNS timestamp with time zone AS $$ BEGIN RETURN date_trunc('quarter', dt); END; $$ LANGUAGE plpgsql; CREATE FUNCTION quarter_end(dt timestamp with time zone) RETURNS timestamp with time zone AS $$ BEGIN RETURN date_trunc('quarter', dt) + interval '3 month' - interval '1 microsecond'; END; $$ LANGUAGE plpgsql;
- SQL Server
CREATE FUNCTION quarter_begin (@dt DATETIME ) RETURNS DATETIME BEGIN RETURN DATEADD (qq, DATEDIFF (qq, 0, @dt), 0) END GO CREATE FUNCTION quarter_end (@dt DATETIME ) RETURNS DATETIME BEGIN RETURN DATEADD ( ms , -3 , DATEADD(mm, 3, dbo.quarter_begin(@dt)) ); END GO
You can use similar auxiliary functions for other periods—most of them will be less complex than the examples above, especially when using than or greater equal to (
>=) and less than (
<) conditions instead of the between operator. Of course you could calculate the boundary dates in your application if you wish.
Tweet this tip
Write queries for continuous periods as explicit range condition. Do this even for a single day—e.g., for the Oracle database:
sale_date >= TRUNC(sysdate) AND sale_date < TRUNC(sysdate + INTERVAL '1' DAY)
Another common obfuscation is to compare dates as strings as shown in the following PostgreSQL example:
SELECT ... FROM sales WHERE
TO_CHAR(sale_Date, 'YYYY-MM-DD') = '1970-01-01'
The problem is, again, converting
DATE_COLUMN. Such conditions are often created in the belief that you cannot pass different types than numbers and strings to the database. Bind parameters, however, support all data types. That means you can for example use a
java.util.Date object as bind parameter. This is yet another benefit of bind parameters.
If you cannot do that, you just have to convert the search term instead of the table column:
SELECT ... FROM sales WHERE
sale_date = TO_DATE('1970-01-01', 'YYYY-MM-DD')
This query can use a straight index on
SALE_DATE. Moreover it converts the input string only once. The previous statement must convert all dates stored in the table before it can compare them against the search term.
Whatever change you make—using a bind parameter or converting the other side of the comparison—you can easily introduce a bug if
SALE_DATE has a time component. You must use an explicit range condition in that case:
SELECT ... FROM sales WHERE sale_date >= TO_DATE('1970-01-01', 'YYYY-MM-DD') AND sale_date < TO_DATE('1970-01-01', 'YYYY-MM-DD') + INTERVAL '1' DAY
Always consider using an explicit range condition when comparing dates.