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We discuss the largest and most homogeneous spectroscopic dataset of field RRLyrae variables (RRLs) available to date. We estimated abundances using bothhigh-resolution and low-resolution ({\Delta S} method) spectra for fundamental(RRab) and first overtone (RRc) RRLs. The iron abundances for 7,941 RRLs weresupplemented with similar literature estimates available, ending up with 9,015RRLs (6,150 RRab, 2,865 RRc). The metallicity distribution shows a mean valueof <[Fe/H]> = -1.51\pm0.01, and {\sigma}(standard deviation)= 0.41 dex with along metal-poor tail approaching [Fe/H] = -3 and a sharp metal-rich tailapproaching solar iron abundance. The RRab variables are more metal-rich(<[Fe/H]>ab = -1.48\pm0.01, {\sigma} = 0.41 dex) than RRc variables (<[Fe/H]>c= -1.58\pm0.01, {\sigma} = 0.40 dex). The relative fraction of RRab variablesin the Bailey diagram (visual amplitude vs period) located along theshort-period (more metal-rich) and the long-period (more metal-poor) sequencesare 80% and 20\%, while RRc variables display an opposite trend, namely 30\%and 70\%. We found that the pulsation period of both RRab and RRc variablessteadily decreases when moving from the metal-poor to the metal-rich regime.The visual amplitude shows the same trend, but RRc amplitudes are almost twotimes more sensitive than RRab amplitudes to metallicity. We also investigatedthe dependence of the population ratio (Nc/Ntot) of field RRLs on themetallicity and we found that the distribution is more complex than in globularclusters. The population ratio steadily increases from ~0.25 to ~0.36 in themetal-poor regime, it decreases from ~0.36 to ~0.18 for -1.8 < [Fe/H] < -0.9and it increases to a value of ~0.3 approaching solar iron abundance.

On the use of field RR Lyrae as Galactic probes: IV. New insights into and around the Oosterhoff dichotomy