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We collected a large data set of field RR Lyrae stars (RRLs) by usingcatalogues already available in the literature and Gaia DR2. We estimated theiron abundances for a sub-sample of 2,382 fundamental RRLs ($\Delta$S method:CaIIK, H$\beta$, H$\gamma$ and H$\delta$ lines) for which are publiclyavailable medium-resolution SDSS-SEGUE spectra. We also included similarestimates available in the literature ending up with the largest and mosthomogeneous spectroscopic data set ever collected for RRLs (2,903). Themetallicity scale was validated by using iron abundances based on highresolution spectra for a fundamental field RRL (V~Ind), for which we collectedX-shooter spectra covering the entire pulsation cycle. The peak([Fe/H]=-1.59$\pm$0.01) and the standard deviation ($\sigma$=0.43 dex) of themetallicity distribution agree quite well with similar estimates available inthe literature. The current measurements disclose a well defined metal-richtail approaching Solar iron abundance. The spectroscopic sample plotted in theBailey diagram (period vs luminosity amplitude) shows a steady variation whenmoving from the metal-poor ([Fe/H]=-3.0/-2.5) to the metal-rich([Fe/H]=-0.5/0.0) regime. The smooth transition in the peak of the perioddistribution as a function of the metallicity strongly indicates that thelong-standing problem of the Oosterhoff dichotomy among Galactic globulars isthe consequence of the lack of metal-intermediate clusters hosting RRLs. Wealso found that the luminosity amplitude, in contrast with period, does notshow a solid correlation with metallicity. This suggests thatperiod-amplitude-metallicity relations should be cautiously treated.

On the use of field RR Lyrae as Galactic probes: I. The Oosterhoff dichotomy based on fundamental variables