  We discuss the largest and most homogeneous spectroscopic dataset of field RR
Lyrae variables (RRLs) available to date. We estimated abundances using both
high-resolution and low-resolution ({\Delta S} method) spectra for fundamental
(RRab) and first overtone (RRc) RRLs. The iron abundances for 7,941 RRLs were
supplemented with similar literature estimates available, ending up with 9,015
RRLs (6,150 RRab, 2,865 RRc). The metallicity distribution shows a mean value
of <[Fe/H]> = -1.51\pm0.01, and {\sigma}(standard deviation)= 0.41 dex with a
long metal-poor tail approaching [Fe/H] = -3 and a sharp metal-rich tail
approaching 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 variables
in the Bailey diagram (visual amplitude vs period) located along the
short-period (more metal-rich) and the long-period (more metal-poor) sequences
are 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 variables
steadily decreases when moving from the metal-poor to the metal-rich regime.
The visual amplitude shows the same trend, but RRc amplitudes are almost two
times more sensitive than RRab amplitudes to metallicity. We also investigated
the dependence of the population ratio (Nc/Ntot) of field RRLs on the
metallicity and we found that the distribution is more complex than in globular
clusters. The population ratio steadily increases from ~0.25 to ~0.36 in the
metal-poor regime, it decreases from ~0.36 to ~0.18 for -1.8 < [Fe/H] < -0.9
and it increases to a value of ~0.3 approaching solar iron abundance.
