Query Plan

Have a question regarding when a Query Plan uses an Index.

I have a basic statement

This query uses the Index Scan:
explain analyse select * from tablea where columna_id < 57

This query uses Seq Scan:
explain analyse select * from tablea where columna_id < 58

There are about 5000 records in this table, and columna_id is the primary
key, which is Indexed... And recently recreated.

My question is why is the Plan not using the index on the second occasions?

Theo

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