Some questions on KEYS

Hi :

In the PostgreSQL 7.2.1 SQL Reference manual I
encountered the following syntax under the "ALTER
TABLE" section.

ALTER TABLE table
ADD <table constraint definition>

ALTER TABLE [ ONLY ] table [ * ]
ADD [ COLUMN ] <column> <type> [
<column_constraint> [ ... ] ]

1) Is <table constraint definition> similar to
<column_constraint>?

2) Is the column <column_constaint> in ALTER TABLE the
same the <column_constraint> found under the section
"CREATE TABLE" ?

3) If a table w/ primary doesn't have any index. Does
it perform a sequential scan on the key columns before
inserting a new record?

4) If primary keys utilizes index, is a multi-column
index on multi-columned key a better implementation
over creating a seperate index for each column in the
primary key?

Thank you in advance,

ludwig

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