Function to estimate BTREE index size based on the table and column(s)

estimate.sql

  CREATE OR REPLACE FUNCTION f_index_minimum_size(_tbl regclass, _cols VARIADIC text[], OUT estimated_minimum_size bigint)
  LANGUAGE plpgsql AS
$func$
DECLARE
   _missing_column text;
BEGIN

-- assert
SELECT i.attname
FROM   unnest(_cols) AS i(attname)
LEFT   JOIN pg_catalog.pg_attribute a ON a.attname = i.attname
                                     AND a.attrelid = _tbl
WHERE  a.attname IS NULL
INTO   _missing_column;

IF FOUND THEN
   RAISE EXCEPTION 'Table % has no column named %', _tbl, quote_ident(_missing_column);
END IF;


SELECT INTO estimated_minimum_size
       COALESCE(1 + ceil(reltuples/trunc((blocksize-page_overhead)/(4+tuple_size)))::int, 0) * blocksize -- AS estimated_minimum_size
FROM  (
   SELECT maxalign, blocksize, reltuples, fillfactor, page_overhead
        , (maxalign  -- up to 16 columns, else nullbitmap may force another maxalign step
         + CASE WHEN datawidth <= maxalign  THEN maxalign
                WHEN datawidth%maxalign = 0 THEN datawidth
                ELSE                            (datawidth + maxalign) - datawidth%maxalign END  -- add padding to the data to align on MAXALIGN
          ) AS tuple_size
   FROM  (
      SELECT c.reltuples, count(*)
           , 90 AS fillfactor
           , current_setting('block_size')::bigint AS blocksize
           , CASE WHEN version() ~ '64-bit|x86_64|ppc64|ia64|amd64|mingw32'  -- MAXALIGN: 4 on 32bits, 8 on 64bits
                  THEN 8 ELSE 4 END AS maxalign
           , 40 AS page_overhead  -- 24 bytes page header + 16 bytes "special space"
           -- avg data width without null values
           , sum(ceil((1-COALESCE(s.null_frac, 0)) * COALESCE(s.avg_width, 1024))::int) AS datawidth  -- ceil() because avg width has a low bias
      FROM   pg_catalog.pg_class     c
      JOIN   pg_catalog.pg_attribute a ON a.attrelid = c.oid
      JOIN   pg_catalog.pg_stats     s ON s.schemaname = c.relnamespace::regnamespace::text
                                      AND s.tablename  = c.relname
                                      AND s.attname    = a.attname
      WHERE  c.oid = _tbl
      AND    a.attname = ANY(_cols) --  all exist, verified above
      GROUP  BY 1
      ) sub1
   ) sub2;
END
$func$;

usage.sql

SELECT pg_size_pretty(f_index_minimum_size('user_post_events', 'user_id', 'post_id'));