Can't count() a PostgreSql table

I have a moderately large PostgreSql table (~20M rows) with one index, receiving a small amount of traffic (1-2 inserts/sec). When I try:

SELECT Count(*) FROM MyTable

The query never returns. With an index on the table, shouldn't it be fulfillable from stats?

What could be the problem, and/or how can I diagnose what's going on? Am running 9.6.15.

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