When we optimize clients’ SQL queries I pretty often see a queries with SQL_CALC_FOUND_ROWS option used. Many people think, that it is faster to use this option than run two separate queries: one – to get a result set, another – to count total number of rows. In this post I’ll try to check, is [...]
Do you always need index on WHERE column ?
August 28, 2007 By 16 Comments
I believe we wrote about this before, but this topic popups again and again. Today I’ve read opinion that if we have clause WHERE has_something=1 we should have index on column `has_something` (the column has two values 0 and 1). In reality the right answer is not so simple.
Redundant index is not always bad
August 28, 2007 By 7 Comments
About year ago Peter wrote about redundant indexes and mentioned sometimes it is good to leave two indexes, even one is first part of another. I’m speaking about BTREE indexes, for example, KEY (A), and KEY (A,B). From SQL point of view KEY(A) is not needed, as for queries like WHERE A=5 the index (A,B) [...]
