This answer shows a great way to convert multi-part to single-part.
Example for converting multilinestring to linestring with SRID 3857:
Check your table if there is any feature with more than 1 part:
SELECT COUNT(CASE WHEN ST_NumGeometries(geom) > 1 THEN 1 END) AS multi_geom,
COUNT(geom) AS total_geom
If you see "multi_geom" in the result of the above statement is 0, it's safe to proceed to the next step. Otherwise, when multi_geom is larger than 0, you have features in my_table that can not convert safely to single part, you'll have to resolve those problem by hand before proceeding.