Index not always being used

I’ve got a table with approximately 5.5 million rows, and one column that is frequently searched is of type varchar(50)  and we just put a trigram index on it. Now, if we search “where bld_city = ‘baskingridge’”, explain says a sequential table scan is going to be used, but if I change the query to use “where bold_city like ‘basking%’”, then explain correctly shows that a bitmap index scan will be performed. Why would these two forms use different approaches? I’m a bit confused.
—
Jay

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