Setting db_index=False on a field doesn't actually remove the index

[Craig de Stigter]

1. Make a field with db_index=True (for ForeignKey, django sets this by default)
2. Make a migration. This creates an index:

CREATE INDEX "a_c_b_id_5a026661" ON "a_c" ("b_id");

3. Explicitly set db_index=False and create another migration
4. Check sqlmigrate for the new migration. Index is never actually removed:

BEGIN;
--
-- Alter field b on c
--
COMMIT;

One note: This is with postgres settings. I'm not sure if it's db-specific.

[Craig de Stigter]

A tiny test project: ​https://github.com/craigds/django-28339-testproject

Output with that project:

$ ./tp3/manage.py sqlmigrate a 0001
BEGIN;
--
-- Create model B
--
CREATE TABLE "a_b" ("id" serial NOT NULL PRIMARY KEY);
--
-- Create model C
--
CREATE TABLE "a_c" ("id" serial NOT NULL PRIMARY KEY, "b_id" integer NOT NULL);
ALTER TABLE "a_c" ADD CONSTRAINT "a_c_b_id_5a026661_fk_a_b_id" FOREIGN KEY ("b_id") REFERENCES "a_b" ("id") DEFERRABLE INITIALLY DEFERRED;
CREATE INDEX "a_c_b_id_5a026661" ON "a_c" ("b_id");
COMMIT;

$ ./tp3/manage.py sqlmigrate a 0002
BEGIN;
--
-- Alter field b on c
--
COMMIT;

[Craig de Stigter]

Turns out, it's not just ForeignKey. Indices are not dropped on other fields too:

$ ./tp3/manage.py sqlmigrate a 0003
BEGIN;
--
-- Add field charfield to b
--
ALTER TABLE "a_b" ADD COLUMN "charfield" varchar(1) DEFAULT '' NOT NULL;
ALTER TABLE "a_b" ALTER COLUMN "charfield" DROP DEFAULT;
--
-- Add field intfield to b
--
ALTER TABLE "a_b" ADD COLUMN "intfield" integer DEFAULT 0 NOT NULL;
ALTER TABLE "a_b" ALTER COLUMN "intfield" DROP DEFAULT;
CREATE INDEX "a_b_charfield_a5049317" ON "a_b" ("charfield");
CREATE INDEX "a_b_charfield_a5049317_like" ON "a_b" ("charfield" varchar_pattern_ops);
CREATE INDEX "a_b_intfield_b730ef11" ON "a_b" ("intfield");
COMMIT;

$ ./tp3/manage.py sqlmigrate a 0004
BEGIN;
--
-- Alter field charfield on b
--
--
-- Alter field intfield on b
--
COMMIT;