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I have created table name as d with ID column as primary key and then just inserted records as shown in output, but after fetching all records this output still displayed same as order in which records are inserted. but output as a see now not in ordered form.
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PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. The index covers the columns that make up the primary key or unique constraint (a multicolumn index, if appropriate), and is the mechanism that enforces the constraint.
The solidDB ® server creates a primary key index automatically based on the field or fields of the primary key. A primary key index, like any index, speeds up access to data in the table. Unlike other indexes, however, a primary key index also controls the order in which records are stored in the database.
Yes a primary key is always an index. If you don't have any other clustered index on the table, then it's easy: a clustered index makes a table faster, for every operation. YES!
Index at the target of a foreign key Such constraints are implemented with unique indexes in PostgreSQL. Consequently, the target side of a foreign key is automatically indexed. This is required so that there is always a well-defined row to which the foreign key points.
PostgreSQL automatically creates an index for each unique constraint and primary key constraint to enforce uniqueness. Thus, it is not necessary to create an index explicitly for primary key columns. (See CREATE INDEX for more information.)
Source: Docs
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