How to checking the existance of constraints for a table?

Hello,

I have a sample table describe as follows, anyone knows how to checking the
existence of a constraint?

oviddb=# \d myTable

Column | Type | Modifiers
---------+----------+-----------
orderid | smallint | not null
modelid | smallint | not null

Indexes:
"mytable_orderid_key" UNIQUE, btree (orderid)

For example, how to know that myTable has a constraint with name
"mytable_orderid_key"?
I have this problem because when doing migration script, some old table may
have this constraint or may not have. And after migration, we want to add
this constraint:

If I just run the command:
alter table myTable add Unique (orderid) in the migration script, when the
constraint exits, a new index will be created as
"mytable_orderid_key1" UNIQUE, btree (orderid)

So, it causes some duplication.
oviddb=# \d myTable

Column | Type | Modifiers
---------+----------+-----------
orderid | smallint | not null
modelid | smallint | not null

Indexes:
"mytable_orderid_key" UNIQUE, btree (orderid)
"mytable_orderid_key1" UNIQUE, btree (orderid)

My question becomes how to write sql statement with following semantecs

if constraint unique (orderid) exists
do nothing
else
alter table myTable add Unique (orderid)

Thank you so much!
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