The UV background photoionization rate at 2.3 < z < 4.6 as measured from the Sloan Digital Sky Survey
(Abridged) We present the results from the largest investigation to date of the proximity effect in the HI Lyalpha forest, using the fifth SDSS data release. The sample consists of 1733 QSOs at redshifts z>2.3 and S/N>10. We adopted the flux statistic to infer the evolution of the HI effective optical depth in the Lyalpha forest between 2<z<4.5, finding very good agreement with results from high-resolution QSO samples. We compared the average opacity close to the quasar emission with its expected behavior in the Lyalpha forest and estimated the signature of the proximity effect towards individual objects at high significance in about 98% of the QSOs. Dividing the whole sample of objects in eight subsets according to their emission redshift, we inferred the proximity effect strength distribution (PESD) on each of them finding in all cases a prominent peak and an extending tail towards values associated to a weak effect. We provide for the first time observational evidence for an evolution in the asymmetry of the PESD with redshift. Adopting the modal values of the PESDs as our best and unbiased estimates of the UV background photoionization rate (Gamma_HI), we determine its evolution within the range 2.3<z<4.6. Our measurements do not show any significant decline towards high redshift and are located at Log(Gamma_HI)=-11.78\pm0.07 in units of s^-1. We decompose the observed photoionization rate into two major contributors: quasar and star-forming galaxies. By modeling the quasar contribution with different luminosity functions we estimated their contribution to Gamma_HI, thus putting a constraint on the residuals. We conclude that independently of the assumed luminosity function, stars are dominating the UVB at z>3.