Re: ***SPAM*** Re: Help with large delete

Perry Smith <pedz(at)easesoftware(dot)com> writes:
> On Apr 16, 2022, at 10:33, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> The most obvious question is do you have an index on the referencing
>> column. PG doesn't require one to exist to create an FK; but if you
>> don't, deletes of referenced rows had better be uninteresting to you
>> performance-wise, because each one will cause a seqscan.

> For Tom’s question, here is the description of the table:

> psql -c '\d dateien' find_dups
> Table "public.dateien"
> Column | Type | Collation | Nullable | Default
> ------------+--------------------------------+-----------+----------+-------------------------------------
> id | bigint | | not null | nextval('dateien_id_seq'::regclass)
> basename | character varying | | not null |
> parent_id | bigint | | |
> dev | bigint | | not null |
> ftype | character varying | | not null |
> uid | bigint | | not null |
> gid | bigint | | not null |
> ino | bigint | | not null |
> mode | bigint | | not null |
> mtime | timestamp without time zone | | not null |
> nlink | bigint | | not null |
> size | bigint | | not null |
> sha1 | character varying | | |
> created_at | timestamp(6) without time zone | | not null |
> updated_at | timestamp(6) without time zone | | not null |
> Indexes:
> "dateien_pkey" PRIMARY KEY, btree (id)
> "unique_dev_ino_for_dirs" UNIQUE, btree (dev, ino) WHERE ftype::text = 'directory'::text
> "unique_parent_basename" UNIQUE, btree (COALESCE(parent_id, '-1'::integer::bigint), basename)
> Foreign-key constraints:
> "fk_rails_c01ebbd0bf" FOREIGN KEY (parent_id) REFERENCES dateien(id) ON DELETE CASCADE
> Referenced by:
> TABLE "dateien" CONSTRAINT "fk_rails_c01ebbd0bf" FOREIGN KEY (parent_id) REFERENCES dateien(id) ON DELETE CASCADE

Yeah. So if you want to make deletes on this table not be unpleasantly
slow, you need an index on the parent_id column, and you don't have one.

(unique_parent_basename doesn't help, because with that definition it's
useless for looking up rows by parent_id.)

regards, tom lane