8.1.3 Optimisation Options
The ‘-Olevel’ option sets
find's optimisation level
to level. The default optimisation level is 1.
At certain optimisation levels,
find reorders tests to speed up
execution while preserving the overall effect; that is, predicates
with side effects are not reordered relative to each other. The
optimisations performed at each optimisation level are as follows.
- Currently equivalent to optimisation level 1.
- This is the default optimisation level and corresponds to the
traditional behaviour. Expressions are reordered so that tests based
only on the names of files (for example‘ -name’ and
‘-regex’) are performed first.
- Any ‘-type’ or ‘-xtype’ tests are performed after any tests
based only on the names of files, but before any tests that require
information from the inode. On many modern versions of Unix, file
types are returned by
readdir() and so these predicates are
faster to evaluate than predicates which need to stat the file first.
If you use the ‘-fstype FOO’ predicate and specify a filsystem
type ‘FOO’ which is not known (that is, present in
/etc/mtab) at the time
find starts, that predicate is
equivalent to ‘-false’.
- At this optimisation level, the full cost-based query optimiser is
enabled. The order of tests is modified so that cheap (i.e., fast)
tests are performed first and more expensive ones are performed later,
if necessary. Within each cost band, predicates are evaluated earlier
or later according to whether they are likely to succeed or not. For
‘-o’, predicates which are likely to succeed are evaluated
earlier, and for ‘-a’, predicates which are likely to fail are