InnoDB is similar to SQLite all tables are clustered indexes, if you don't have suitable primary key it creates a hidden one similar to rowid in SQLite.
MSSQL and Oracle give you a choice for table storage to either have unordered heaps or ordered b-trees, depending on whether a clustered index is defined.
PG only has unordered heaps, indexes are always secondary data structures. PG could really use true clustered indexes as an option.
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Clustered indexes in MSSQL Server, Index Organized Tables in Oracle, and any table in MySQL using InnoDB as storage engine.
InnoDB is similar to SQLite all tables are clustered indexes, if you don't have suitable primary key it creates a hidden one similar to rowid in SQLite.
MSSQL and Oracle give you a choice for table storage to either have unordered heaps or ordered b-trees, depending on whether a clustered index is defined.
PG only has unordered heaps, indexes are always secondary data structures. PG could really use true clustered indexes as an option.