  We report the first molecular line survey of Supernova 1987A in the
millimetre wavelength range. In the ALMA 210--300 and 340--360 GHz spectra, we
detected cold (20--170 K) CO, 28SiO, HCO+ and SO, with weaker lines of 29SiO
from ejecta. This is the first identification of HCO+ and SO in a young
supernova remnant. We find a dip in the J=6--5 and 5--4 SiO line profiles,
suggesting that the ejecta morphology is likely elongated. The difference of
the CO and SiO line profiles is consistent with hydrodynamic simulations, which
show that Rayleigh-Taylor instabilities cause mixing of gas, with heavier
elements much more disturbed, making more elongated structure. We obtained
isotopologue ratios of 28SiO/29SiO>13, 28SiO/30SiO>14, and 12CO/13CO>21, with
the most likely limits of 28SiO/29SiO>128, 28SiO/30SiO>189. Low 29Si and 30Si
abundances in SN 1987A are consistent with nucleosynthesis models that show
inefficient formation of neutron-rich isotopes in a low metallicity
environment, such as the Large Magellanic Cloud. The deduced large mass of HCO+
(~5x10^-6 Msun) and small SiS mass (<6x10^-5 Msun) might be explained by some
mixing of elements immediately after the explosion. The mixing might have
caused some hydrogen from the envelope to sink into carbon and oxygen-rich
zones after the explosion, enabling the formation of a substantial mass of
HCO+. Oxygen atoms may have penetrated into silicon and sulphur zones,
suppressing formation of SiS. Our ALMA observations open up a new window to
investigate chemistry, dynamics and explosive-nucleosynthesis in supernovae.
