DDL generation fixes fpr oracle platform (fix drop default, do "drop if exists" with error handlers)

ebean-ddl-generator/src/main/java/io/ebeaninternal/dbmigration/ddlgeneration/platform/OracleDdl.java

@@ -20,6 +20,8 @@ public class OracleDdl extends PlatformDdl {
     this.columnSetNotnull = "not null";
     this.columnSetNull = "null";
     this.columnSetDefault = "default";
+    this.columnDropDefault = "default null"; // dropping default is not supported. See:
+    // https://stackoverflow.com/questions/8481532/how-do-i-remove-the-default-value-from-a-column-in-oracle
     this.identitySuffix = " generated by default as identity";
   }
 
@@ -33,6 +35,21 @@ public class OracleDdl extends PlatformDdl {
     }
   }
 
+  @Override
+  public String dropSequence(String sequenceName) {
+    return ignoreError(-2289, super.dropSequence(sequenceName));
+  }
+
+  @Override
+  public String alterTableDropConstraint(String tableName, String constraintName) {
+    return ignoreError(-2443, super.alterTableDropConstraint(tableName, constraintName));
+  }
+
+  @Override
+  public String alterTableDropUniqueConstraint(String tableName, String uniqueConstraintName) {
+    return ignoreError(-2443, super.alterTableDropUniqueConstraint(tableName, uniqueConstraintName));
+  }
+
   @Override
   protected void appendForeignKeyOnUpdate(StringBuilder buffer, ConstraintMode mode) {
     // do nothing, no on update clause for oracle
@@ -57,4 +74,21 @@ public class OracleDdl extends PlatformDdl {
     return asIdentityStandardOptions(columnDefn, identity);
   }
 
+  /**
+   * generates anonymous pl/sql block that catches expected error so that we can
+   * effectively do a 'drop if exists'
+   */
+  private String ignoreError(int errorNr, String statement) {
+    StringBuilder sb = new StringBuilder();
+    sb.append("delimiter $$\n")
+        .append("declare\n")
+        .append("  expected_error exception;\n")
+        .append("  pragma exception_init(expected_error, ").append(errorNr).append(");\n")
+        .append("begin\n")
+        .append("  execute immediate '").append(statement).append("';\n")
+        .append("exception\n")
+        .append("  when expected_error then null;\n")
+        .append("end;\n$$");
+    return sb.toString();
+  }
 }