Parameter:

Name.................: _OPTIM_PEEK_USER_BINDS
Values...............: TRUE/FALSE
Default value........: TRUE
Initial Release......: 9.0.1
Scope................: Instance/Session

Explanation:

Until Oracle 9.0.1, values of bind variables are known in the PARSE phase. Since it's not known, it's not possible to generate execution plans according to bind values.

With 9i and onwards, Oracle peeks the values of bind variables in the FIRST PARSE phase and generates execution plans according to the values in this first PARSE. If subsequent bind values are skewed, then execution plans may not be optimal for the subsequent binds.