sql: SHOW CREATE TABLE treated unique constraints like unique indexes

```
root@localhost:26257/test> CREATE TABLE t (i INT PRIMARY KEY, j INT);
CREATE TABLE
root@localhost:26257/test> ALTER TABLE t ADD CONSTRAINT c UNIQUE (j);
ALTER TABLE
root@localhost:26257/test> CREATE UNIQUE INDEX idx ON t (j);
CREATE INDEX
```

Before:
```
root@localhost:26257/test> SHOW CREATE TABLE t;
  table_name |               create_statement
-------------+------------------------------------------------
  t          | CREATE TABLE public.t (
             |     i INT8 NOT NULL,
             |     j INT8 NULL,
             |     CONSTRAINT "primary" PRIMARY KEY (i ASC),
             |     UNIQUE INDEX c (j ASC),
             |     UNIQUE INDEX idx (j ASC),
             |     FAMILY "primary" (i, j)
             | )
```

After:
```
[email protected]:55792/movr> SHOW CREATE TABLE t;
  table_name |               create_statement
-------------+------------------------------------------------
  t          | CREATE TABLE public.t (
             |     i INT8 NOT NULL,
             |     j INT8 NULL,
             |     CONSTRAINT "primary" PRIMARY KEY (i ASC),
             |     UNIQUE INDEX idx (j ASC),
             |     FAMILY "primary" (i, j),
             |     CONSTRAINT c UNIQUE (j)
             | )
```

Release note (bug fix): UNIQUE constraints were displayed as UNIQUE INDEX
entries in SHOW CREATE TABLE.

pkg/sql/show_create.go

@@ -149,6 +149,10 @@ func ShowCreateTable(
 			// clauses as well.
 			includeInterleaveClause = true
 		}
+		// Don't show indexes which represent unique constraints.
+		if idx.IsUnique() && !idx.IsCreatedExplicitly() {
+			continue
+		}
 		if !idx.Primary() && includeInterleaveClause {
 			// Showing the primary index is handled above.
 

pkg/sql/show_create_clauses.go

@@ -601,6 +601,28 @@ func showConstraintClause(
 			f.WriteString(" NOT VALID")
 		}
 	}
+	for _, idx := range desc.AllIndexes() {
+		if !idx.IsUnique() || idx.IsCreatedExplicitly() || idx.Primary() {
+			continue
+		}
+		f.WriteString(",\n\t")
+		if len(idx.GetName()) > 0 {
+			f.WriteString("CONSTRAINT ")
+			formatQuoteNames(&f.Buffer, idx.GetName())
+			f.WriteString(" ")
+		}
+		f.WriteString("UNIQUE ")
+		f.WriteString("(")
+		startIdx := idx.ExplicitColumnStartIdx()
+		for i := startIdx; i < idx.NumColumns(); i++ {
+			if i > startIdx {
+				f.WriteString(", ")
+			}
+			n := idx.GetColumnName(i)
+			f.FormatNameP(&n)
+		}
+		f.WriteString(")")
+	}
 	f.WriteString("\n)")
 	return nil
 }