pref: refactor index item to save memory useage

[bigboss2063]

No description provided.

[codecov]

Codecov Report

Merging #434 (3c9cfac) into master (03100c9) will increase coverage by 1.40%.
The diff coverage is 85.61%.

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
+ Coverage   65.50%   66.90%   +1.40%     
==========================================
  Files          33       33              
  Lines        5829     5820       -9     
==========================================
+ Hits         3818     3894      +76     
+ Misses       1682     1583      -99     
- Partials      329      343      +14     

Files Changed	Coverage Δ
db.go	60.82% <73.80%> (+0.24%)	⬆️
bptree.go	81.28% <75.00%> (ø)
tx_tree.go	27.89% <75.86%> (+10.02%)	⬆️
tx.go	72.15% <98.27%> (+0.38%)	⬆️
btree.go	92.00% <100.00%> (ø)
record.go	100.00% <100.00%> (ø)