We collected a large data set of field RR Lyrae stars ( RRLs ) by using catalogues already available in the literature and Gaia DR2 .
We estimated the iron abundances for a sub-sample of 2,382 fundamental RRLs ( \Delta S method : Ca II K , H \beta , H \gamma and H \delta lines ) for which are publicly available medium-resolution SDSS-SEGUE spectra .
We also included similar estimates available in the literature ending up with the largest and most homogeneous spectroscopic data set ever collected for RRLs ( 2,903 ) .
The metallicity scale was validated by using iron abundances based on high resolution spectra for a fundamental field RRL ( V Ind ) , for which we collected X-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 the metallicity distribution agree quite well with similar estimates available in the literature .
The current measurements disclose a well defined metal-rich tail approaching Solar iron abundance .
The spectroscopic sample plotted in the Bailey diagram ( period vs luminosity amplitude ) shows a steady variation when moving 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 period distribution as a function of the metallicity strongly indicates that the long-standing problem of the Oosterhoff dichotomy among Galactic globulars is the consequence of the lack of metal-intermediate clusters hosting RRLs .
We also found that the luminosity amplitude , in contrast with period , does not show a solid correlation with metallicity .
This suggests that period-amplitude-metallicity relations should be cautiously treated .