Documentation Home
MySQL 5.7 Reference Manual
Related Documentation Download this Manual
PDF (US Ltr) - 33.9Mb
PDF (A4) - 34.0Mb
PDF (RPM) - 33.2Mb
EPUB - 8.5Mb
HTML Download (TGZ) - 8.2Mb
HTML Download (Zip) - 8.2Mb
HTML Download (RPM) - 7.1Mb
Eclipse Doc Plugin (TGZ) - 9.0Mb
Eclipse Doc Plugin (Zip) - 11.1Mb
Man Pages (TGZ) - 219.4Kb
Man Pages (Zip) - 322.3Kb
Info (Gzip) - 3.2Mb
Info (Zip) - 3.2Mb
Excerpts from this Manual

MySQL 5.7 Reference Manual  /  ...  /  Using Foreign Keys

8.3.3 Using Foreign Keys

If a table has many columns, and you query many different combinations of columns, it might be efficient to split the less-frequently used data into separate tables with a few columns each, and relate them back to the main table by duplicating the numeric ID column from the main table. That way, each small table can have a primary key for fast lookups of its data, and you can query just the set of columns that you need using a join operation. Depending on how the data is distributed, the queries might perform less I/O and take up less cache memory because the relevant columns are packed together on disk. (To maximize performance, queries try to read as few data blocks as possible from disk; tables with only a few columns can fit more rows in each data block.)


User Comments
Sign Up Login You must be logged in to post a comment.