SQL group by help

with rack_display as (
    select sr.ts rack_ts
        , sr.rack_id
        , r.rack_barcode
        , 1 as row_pos
        , max(case when rack_well = 0 then 'A1: '||sample_barcode end) as col1
        , max(case when rack_well = 1 then 'A2: '||sample_barcode end) as col2
        , max(case when rack_well = 2 then 'A3: '||sample_barcode end) as col3
        , max(case when rack_well = 3 then 'A4: '||sample_barcode end) as col4
        , max(case when rack_well = 4 then 'A5: '||sample_barcode end) as col5
        , max(case when rack_well = 5 then 'A6: '||sample_barcode end) as col6
        from rack r
                        , sample_rack sr
            , sample s
        where r.rack_id = sr.rack_id
           and sr.sample_id = s.sample_id
           and sr.rack_well < 6
        group by sr.ts, sr.rack_id, r.rack_barcode, row_pos
    union all
    select sr.ts rack_ts
        , sr.rack_id
        , r.rack_barcode
        , 2 as row_pos
        , max(case when rack_well = 6 then 'B1: '||sample_barcode end) as col1
        , max(case when rack_well = 7 then 'B2: '||sample_barcode end) as col2
        , max(case when rack_well = 8 then 'B3: '||sample_barcode end) as col3
        , max(case when rack_well = 9 then 'B4: '||sample_barcode end) as col4
        , max(case when rack_well = 10 then 'B5: '||sample_barcode end) as col5
        , max(case when rack_well = 11 then 'B6: '||sample_barcode end) as col6
        from rack r
                        , sample_rack sr
            , sample s
        where r.rack_id = sr.rack_id
           and sr.sample_id = s.sample_id
           and sr.rack_well >= 6
           and sr.rack_well < 12
        group by sr.ts, sr.rack_id, r.rack_barcode, row_pos
    union all

...

)

select * from rack_display order by rack_ts, rack_id, row_pos;

the "union all"s continue for another 6 blocks. reports would filter on rack_id and timestamp. 

if timestamps for each load of a rack were guaranteed to be the same, this would work. however, the "sr.ts" values may vary by a few seconds so there is potential for the "group by" to break. ts differences will be a minimum of 5 minutes for each distinct load of a rack. 

what i think i need is to manufacture a group by column based off rows in "sample_rack" that have "ts" values that are < 1 minute from each other and rack_id is the same.  i'm coming up blank on how to accomplish that though. my first thought was to create an interval of +/- 1 min then find all rows that overlap and assign a group number but i'm not sure how to accomplish that. 

there's also no guarantee an entire rack is full of samples so some "cells" of display might be null. i think that makes the use of tablefunc crosstab a little harder. if i remember correctly, it does not handle missing values well. i'm open to any pivoting strategy.

anyways, i thought i'd reach out for ideas while i do my own digging.

thanks for any input! let me know if i'm not being clear on the problem and desired outcome.