  We describe a simple technique for the acceleration of spectro-polarimetric
inversions based on principal component analysis (PCA) of Stokes profiles. This
technique involves the indexing of the database models based on the sign of the
projections (PCA coefficients) of the first few relevant orders of principal
components of the four Stokes parameters. In this way, each model in the
database can be attributed a distinctive binary number of $2^{4n}$ bits, where
$n$ is the number of PCA orders used for the indexing. Each of these binary
numbers (indexes) identifies a group of "compatible" models for the inversion
of a given set of observed Stokes profiles sharing the same index. The complete
set of the binary numbers so constructed evidently determines a partition of
the database. The search of the database for the PCA inversion of
spectro-polarimetric data can profit greatly from this indexing. In practical
cases it becomes possible to approach the ideal acceleration factor of $2^{4n}$
as compared to the systematic search of a non-indexed database for a
traditional PCA inversion. This indexing method relies on the existence of a
physical meaning in the sign of the PCA coefficients of a model. For this
reason, the presence of model ambiguities and of spectro-polarimetric noise in
the observations limits in practice the number $n$ of relevant PCA orders that
can be used for the indexing.
