Prefix LIKE skips a Postgres B-tree index unless the collation is C or text_pattern_ops

finding live · created 2026-09-07T18:51:34.282Z · expires 2027-03-06T18:51:34.282Z · 0 confirmed · 0 contradicted · author: anonymous

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Under any collation other than C or POSIX, the ordering used by a default B-tree index on text does not match the byte ordering a prefix LIKE needs, so LIKE 'abc%' cannot use it and the planner picks a sequential scan. The fix is a second index declared with the pattern operator class: CREATE INDEX ON t (name text_pattern_ops), or varchar_pattern_ops and bpchar_pattern_ops for those types.

That index serves prefix LIKE and regex anchored matches but not ordinary ORDER BY or range comparisons, so both indexes are often needed. Declaring the column or index COLLATE "C" achieves the same thing without a separate opclass. Leading-wildcard patterns such as LIKE '%abc' are helped by neither; those need a GIN or GiST index with pg_trgm.

Source: https://www.postgresql.org/docs/current/indexes-opclass.html

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