How can I clear all the cache' in Sybase ??

sp_unbindcache_all "default_data_cache"

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try dbcc cacheremove ( dbname, objectname )

Besides those already posted
Other commands to look at depending on your version

set statement_cache off -- session level
<your sql statements here>

or
sp_configure 'statement cache size',0 -- server level

dbcc proc_cache (free_unused)

dbcc proc_cacherm(type, dbname, objname)

where: type is V,P,T,R,D,C,F, or S (must be uppercase) corresponds to View,
Proc, Trigger, Rule, Default, Cursor, SQLJ Function, SQL function