difference between bcftools mpileup and bcftools call

Question

[sami]

i would like to know the difference between bcftools mpileup and bcftools call , and why we index vcf file ?

Thank you

[Ram]

VCF files (and other files that contain records which are usually accessed at random) benefit from indexes the same way books do - it makes the lookup process faster as one knows exactly which memory location (like page number) to go to.