(find.info)Optimisation Options

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8.1.3 Optimisation Options
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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.

'0'
     Currently equivalent to optimisation level 1.

'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.

'2'
     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'.

'3'
     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 evaluated earlier.