We've got a table that has a definition as follows:
CREATE TABLE linking_table (
fk int8 REFERENCES source_table( pk1 ),
PRIMARY KEY( fk1, value )
I would've thought that the multicolumn primary key would behave as a
multicolumn index is supposed to behave per
where the behavior of the index cascades from the left rightward across
any columns specified in WHERE.
But a query like
SELECT COUNT( * ) FROM linking_table WHERE fk = '42';
yields a sequential scan.
If I add an index to fk, then the same query yields an index scan, as I
would expect. Is this because, according to the docs, a primary key "is
merely a combination of UNIQUE and NOT NULL"?
If so, then why do primary keys afford index scans of single columns
specified as primary keys?
This is in postgres 7.4.5, btw.
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