Why PostgreSQL multi-column index not working for first column?

Solution 1:

Thanks to @thorsten-kettner

If fetched records so many rows, will not use index.

I try to use another eq_id with small result sets and index is working:

# select count(*), eq_id from para_data group by eq_id;
 count |      eq_id       
-------+------------------
 52501 | 0100C0130479A5F1
 52348 | 020090120479F281
 52614 | 020280240479A5F1
   559 | 070030210481AEF9
  7013 | 0F0180340479F349
  3910 | 130230120479F281
   229 | 1500C0280479F349
  2014 | 150100340479F349
    86 | 150270330479F349
  3357 | 1600B02E0479F349

# explain analyze verbose SELECT "ch00_status", "ch00_wf_peak_peak" FROM para_data WHERE eq_id = '070030210481AEF9';
                                                                         QUERY PLAN                                                                          
-------------------------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using para_data_eq_id_timestamp_high_idx on public.para_data  (cost=0.42..1146.48 rows=588 width=16) (actual time=0.027..0.325 rows=559 loops=1)
   Output: ch00_status, ch00_wf_peak_peak
   Index Cond: ((para_data.eq_id)::text = '070030210481AEF9'::text)
 Planning Time: 0.049 ms
 Execution Time: 0.350 ms

Thanks to you all.