Is it possible to temporarily disable an index in Postgres?

You can poke the system catalogue to disable an index:

update pg_index set indisvalid = false where indexrelid = 'test_pkey'::regclass

This means that the index won't be used for queries but will still be updated. It's one of the flags used for concurrent index building. Note that I've only done a quick test to see if the index still seems to be updated, caveat emptor.

begin;
drop index foo_ndx;
explain analyze select * from foo;
rollback;

I don't think there is a way to disable just one, though you can do this in a transaction to make recovering from it dead simple. You can also disable indexscan to disable all indices.

Also, make sure you are doing explain analyze on your queries.

I've created (and vetted) a function that does this for a wide variety of circumstances. You definitely need to be careful with it, but it does work.

CREATE OR REPLACE PROCEDURE global.enable_indexes(
    IN schema_name TEXT
,   IN table_name TEXT
,   IN index_name TEXT
,   IN enable_use BOOLEAN
,   IN enable_updates BOOLEAN
,   IN skip_essentials BOOLEAN DEFAULT TRUE
) AS
$$
DECLARE
    each_record RECORD;
BEGIN

/*
USAGE:

schema_name:
- filters down by schema name
- if NULL then does not filter
table_name:
- filters down by table name (careful - not schema qualified)
- if NULL then does not filter
index_name:
- filters down by index name (careful - not schema qualified)
- if NULL then does not filter
enable_use:
- This SETs the index as being available for use.
- If enable_updates is FALSE, then automatically FALSE
enable_updates:
- This SETs the index to be updated (which doesn't imply that it is enabled for use)
- When this was previously FALSE for the given index, then when setting to true will also trigger a rebuild of the index
skip_essentials:
- When this is true, PRIMARY and UNIQUE indexes will not be included in the scope (no changes).
- Optional
*/


IF array_replace(ARRAY[schema_name, table_name, index_name], NULL, '') <@ ARRAY[''] THEN
    RAISE EXCEPTION 'Error: Must specify at least one of schema_name | table_name | index_name';
END IF;

IF enable_updates IS FALSE THEN
    enable_use := FALSE;
    RAISE INFO 'FYI: Because enable_updates is FALSE, then likewise enable_use must be as well';
END IF;

FOR each_record IN
    select
        indexrelid
    ,   (schemaname||'.'||indexname) as index_name
    ,   indisvalid
    ,   indisready
    ,   (schemaname||'.'||tablename) as table_name
    ,   (CASE WHEN indisready IS FALSE and enable_use IS TRUE AND enable_updates IS TRUE THEN TRUE ELSE FALSE END) as needs_rebuilding
    FROM pg_indexes, pg_index
    WHERE
        indexrelid = (schemaname||'.'||indexname)::regclass
    AND case when schema_name <> '' THEN schemaname = schema_name ELSE TRUE END
    AND case when table_name <> '' THEN tablename = table_name ELSE TRUE END
    AND case when index_name <> '' THEN indexname = index_name ELSE TRUE END
    AND case when true THEN least(indisprimary, indisunique) = FALSE ELSE TRUE END
    AND case when skip_essentials THEN least(indisprimary, indisunique) = FALSE ELSE TRUE END
LOOP
    BEGIN

    RAISE INFO 'Set index % to have use % and updates %.'
        ,   each_record.index_name
        ,   (case when each_record.indisvalid AND enable_use THEN 'enabled (not changed)' WHEN NOT each_record.indisvalid AND enable_use THEN 'enabled (changed)' else 'disabled' END)
        ,   (case when each_record.indisready AND enable_updates THEN 'enabled (not changed)' WHEN NOT each_record.indisready AND enable_updates THEN 'enabled (changed)' else 'disabled' END)
    ;

    UPDATE pg_index
    SET
        indisvalid = enable_use
    ,   indisready = enable_updates
    WHERE
        indexrelid = each_record.indexrelid
    ;

    IF each_record.needs_rebuilding THEN
        RAISE INFO '... Reindexing and Analyzing %', each_record.index_name;
        EXECUTE format('REINDEX INDEX %1$s; ANALYZE %2$s;', each_record.index_name, each_record.table_name);
    END IF;

    COMMIT;

    END;

END LOOP;

END
$$
LANGUAGE plpgsql
;