PostgreSQL/PostGIS 9.6 broke my compound index

Question

In PostgreSQL 9.2 I had no problem creating the an index that had both a geography (postGIS) type and and integer as a compound index. But now (9.6) it complains on creation of the index and I don't understand the hint it is providing:

The columns and data are all created properly, Postgres is complaining on the create index.

ERROR: data type integer has no default operator class for access method "gist" 
HINT: You must specify an operator class for the index 
      or define a default operator class for the data type. 
********** Error**********  
ERROR: data type integer has no default operator class for access method "gist" 
SQL state: 42704 
Hint: You must specify an operator class for the index 
      or define a default operator class for the data type.

The schema definition is as follows:

- Table: portal.inventory

-- DROP TABLE portal.inventory;

CREATE TABLE portal.inventory
(
  type character varying,
  pid integer,
  size bigint,
  date timestamp without time zone,
  path character varying,
  outline geography(Polygon,4326)
)
WITH (
  OIDS=FALSE
);
ALTER TABLE portal.inventory
  OWNER TO postgres;

-- Index: portal.inventory_compound_idx

-- DROP INDEX portal.inventory_compound_idx;

CREATE INDEX inventory_compound_idx
  ON portal.inventory
  USING gist
  (outline, pid);

-- Index: portal.inventory_icompound_idx

-- DROP INDEX portal.inventory_icompound_idx;

CREATE INDEX inventory_icompound_idx
  ON portal.inventory
  USING gist
  (pid, outline);

Answer 1

You need to install a specific EXTENSION in your database:

CREATE EXTENSION btree_gist ;

According to PostgreSQL documentation on btree_gist:

btree_gist provides GiST index operator classes that implement B-tree equivalent behavior for the data types int2, int4, int8, float4, float8, numeric, timestamp with time zone, timestamp without time zone, time with time zone, time without time zone, date, interval, oid, money, char, varchar, text, bytea, bit, varbit, macaddr, inet, and cidr.

In general, these operator classes will not outperform the equivalent standard B-tree index methods, and they lack one major feature of the standard B-tree code: the ability to enforce uniqueness. However, they provide some other features that are not available with a B-tree index, as described below. Also, these operator classes are useful when a multicolumn GiST index is needed, wherein some of the columns are of data types that are only indexable with GiST but other columns are just simple data types. Lastly, these operator classes are useful for GiST testing and as a base for developing other GiST operator classes.

(emphasis mine)

btree_gist is part of the standard (current) PostgreSQL installation, so, you don't actually need to install any files in your system.

After installing this extension, you can execute all these instructions on a clean install of PostgreSQL 9.6.2, without a glitch:

-- If there is not there, create extension PostGis as well
CREATE EXTENSION IF NOT EXISTS postgis ;

-- Create Schema `portal`
CREATE SCHEMA IF NOT EXISTS portal ;

And execute all your CREATE statements without a glitch.

CREATE TABLE portal.inventory
(
  type character varying,
  pid integer,
  size bigint,
  date timestamp without time zone,
  path character varying,
  outline geography(Polygon,4326)
);

CREATE INDEX inventory_compound_idx
  ON portal.inventory
  USING gist
  (outline, pid);

CREATE INDEX inventory_icompound_idx
  ON portal.inventory
  USING gist
  (pid, outline);

---

NOTE: This was also needed for version 9.2, according to comment from @Erwin Brandstetter. So, most probably, if you make a dump of the version 9.2 databse, the CREATE EXTENSION btree_gist ; statement should appear.