importccl: Add DROP TABLE [IF EXISTS] support for import pgdump.

[mokaixu]

Previously, whenever a DROP TABLE statement was parsed, an
error was thrown and the import would fail since DROP TABLE
statements were not supported.

Now, when we encounter a DROP TABLE statement for a target table
foo, if foo exists, then we throw an error indicating to the user
to drop the table foo. Otherwise, if foo does not exist, we silently
ignore the DROP statement and proceed with the pgdump import.

Resolves: #53112

Release note: None

[cockroach-teamcity]

This change is 

[adityamaru]

Reviewed 1 of 2 files at r1.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @adityamaru and @mokaixu)

---

pkg/ccl/importccl/read_import_pgdump.go, line 511 at r1 (raw file):

		}
	case *tree.DropTable:
		names := stmt.Names

can we just add a short comment documenting the behavior. If we find a table with the same name in the DB being imported into and public schema then we....else....

---

pkg/ccl/importccl/read_import_pgdump.go, line 525 at r1 (raw file):

				)
				if err != nil {
					return errors.Wrapf(err, `drop table "%s" to proceed with import`, tableName)

nit: drop table foo and then retry the import

---

pkg/ccl/importccl/read_import_pgdump.go, line 910 at r1 (raw file):

				return errors.Errorf("unsupported %T statement: %s", i, i)
			}
		case *tree.DropTable:

can we just add this to the list of tree.CreateTable etc cause it is "handled during schema extraction"

[mokaixu]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @adityamaru)

---

pkg/ccl/importccl/read_import_pgdump.go, line 511 at r1 (raw file):

Previously, adityamaru (Aditya Maru) wrote…

can we just add a short comment documenting the behavior. If we find a table with the same name in the DB being imported into and public schema then we....else....

done

---

pkg/ccl/importccl/read_import_pgdump.go, line 525 at r1 (raw file):

Previously, adityamaru (Aditya Maru) wrote…

nit: drop table foo and then retry the import

done

---

pkg/ccl/importccl/read_import_pgdump.go, line 910 at r1 (raw file):

Previously, adityamaru (Aditya Maru) wrote…

can we just add this to the list of tree.CreateTable etc cause it is "handled during schema extraction"

makes sense, done

[adityamaru]

Reviewed 2 of 2 files at r2.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @adityamaru)

[mokaixu]

bors r=adityamaru

[craig]

Build succeeded:

• GitHub CI (Cockroach)