Double OR operator (||) in bcftools

Question

[scottbrainard]

In the man page for bcftools expressions, the difference between | and || is described as follows:

QUAL>10 | FMT/GQ>10 .. true for sites with QUAL>10 or a sample with GQ>10, but selects only samples with GQ>10

QUAL>10 || FMT/GQ>10 .. true for sites with QUAL>10 or a sample with GQ>10, plus selects all samples at such sites

However, I don't seen any difference in the output from these two commands, which filter out sites with more than 2 alleles, or genotype depth <= 20:

bcftools filter unfiltered.bcf.gz -e 'N_ALT >= 2 | FMT/DP<=20' | bcftools query -l | wc -l  #672
bcftools filter unfiltered.bcf.gz -e 'N_ALT >= 2 || FMT/DP<=20' | bcftools query -l | wc -l  #672

As I understand the manual, the first command should filter out samples with depth <=20. I can separately test to confirm that indeed, there are samples which have read depth <20 at many markers:

bcftools view -H unfiltered.vcf.gz | wc -l #9989 
bcftools filter unfiltered.bcf.gz -e 'FMT/DP<=20' | bcftools view -H | wc -l #97

Any advice on how I am misinterpreting | and || would be appreciated. Thanks!

[scottbrainard]

This question is related to a broader question regarding filtering by sample attributes, instead of site attributes, as described here: https://github.com/samtools/bcftools/issues/1391. Any suggestions on alternative methods for filtering by samples would be appreciated.