>>>>> "Greg" == Greg Stark <st...@mit.edu> writes:

 >> No, because as the pages split, they fill more slowly (because there
 >> are now more pages). So on average in a large randomly filled index,
 >> pages spend more time nearer 50% full than 100% full. This is easy
 >> to demonstrate by creating a table with an indexed float8 column and
 >> adding batches of random() values to it, checking with pgstatindex
 >> at intervals - the average leaf density will rarely exceed 70%.
 >> However, worst case conditions can give lower leaf densities;
 >> obviously the worst case is if the data is loaded in an order and
 >> quantity that just happens to leave every leaf page recently split.

 Greg> btree pages don't split 50/50 either. They split biased to assume
 Greg> the greater side of the split will receive more inserts -- iirc
 Greg> 70/30.

Hmm? The code in _bt_findsplitloc and _bt_checksplitloc doesn't seem to
agree with this.

(Inserting on the high leaf page is a special case, which is where the
fillfactor logic kicks in; that's why sequentially filled indexes are
(by default) 90% full rather than 100%. But other pages split into
roughly equal halves.)

Andrew (irc:RhodiumToad)

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