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From: Quentin de Metz <[email protected]>
To: [email protected]
Subject: Sharp edge when dropping columns
Date: Thu, 14 Mar 2024 19:44:31 +0100
Message-ID: <[email protected]> (raw)

[This is my first message on a PG mailing list, here's hoping I'm not breaching etiquette somehow]

Bonjour,

I'd like to report a sharp edge which caused downtime in my production application: dropping a column which is INCLUDEd in an index drops the index. 

In the past, I have included a field in some of my indexes so that related queries can be performed as an index-only scan. This field was later deprecated. When the related column was ultimately dropped, the performance of the initial queries ground to a halt because the underlying index had been dropped.

I found this behavior unexpected. I think preventing the index from being dropped in some circumstances would make this less surprising. The exact circumstances likely need to be hashed out if there is a consensus that this is a (minor) problem.

Please find below a simple reproduction script:

=# CREATE TABLE test (id bigint, label text);
CREATE TABLE

=# CREATE INDEX pkey_with_label ON test USING btree (id) INCLUDE (label);
CREATE INDEX

=# \d test
Table "public.test"
┌────────┬────────┬───────────┬──────────┬─────────┐
│ Column │ Type   │ Collation │ Nullable │ Default │
├────────┼────────┼───────────┼──────────┼─────────┤
│ id     │ bigint │           │          │         │
│ label  │ text   │           │          │         │
└────────┴────────┴───────────┴──────────┴─────────┘
Indexes:
"pkey_with_label" btree (id) INCLUDE (label)

=# ALTER TABLE test DROP COLUMN label;
ALTER TABLE

=# \d test
Table "public.test"
┌────────┬────────┬───────────┬──────────┬─────────┐
│ Column │ Type   │ Collation │ Nullable │ Default │
├────────┼────────┼───────────┼──────────┼─────────┤
│ id     │ bigint │           │          │         │
└────────┴────────┴───────────┴──────────┴─────────┘

Thank you for reading,

Quentin de Metz

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