Thread

1. Use log_newpage_range in HASH index build

   Kirill Reshke <reshkekirill@gmail.com> — 2025-10-23T19:21:35Z

   There exists an optimization to index creation process, when we omit
   to write any WAL
   for index build. It is currently supported in B Tree, GIN, GiST, spg indexes.
   It works because we do not need to recover anything if index creation
   fails, because if was not used by any query. So, the index can be
   built on-disk, and then, just before making the index alive, we can
   simply log all pages to WAL.
   
   Hash index currently lacks this optimization.
   PFA implementation.
   
   During my testing, I checked the amount of WAL generated by index
   build before and after patch applied. My script was something like:
   
   select pg_current_wal_insert_lsn();
   
   create index on t using hash (i);
   
   select pg_current_wal_insert_lsn();
   
   select pg_lsn_wal_diff(lsn1, lsn2);
   
   Resulting numbers depend on index size, but I got 2.5-3.5 times less
   WAL with this patch and 8 times less WAL with this patch +
   wal_compression=on.
   Index creation time, however, did not change much...
   
   About implementation:
   These are many types of record that can be generated during index build.
   I know for sure these are possible (double-checked using pg_waldump):
   
   SPLIT_COMPLETE
   INSERT
   SPLIT_ALLOCATE_PAGE
   SPLIT_PAGE
   ADD_OVFL_PAGE
   SQUEEZE_PAGE
   INIT_META_PAGE
   INIT_BITMAP_PAGE
   
   
   Looks like  SPLIT_COMPLETE and VACUUM_ONE_PAGE are never generated
   during index build. I'm not sure about  MOVE_PAGE_CONTENTS.
   
   So, implementation is simply pass isbuild flag everywhere something is
   wal-logged. Looks like it is less invasive than alternatives.
   
   -- 
   Best regards,
   Kirill Reshke
   

2. Re: Use log_newpage_range in HASH index build

   lakshmi <lakshmigcdac@gmail.com> — 2025-12-23T11:53:07Z

   Hi Kirill,
   
   I tested your patch on the current master and confirmed the WAL reduction
   during HASH index build.
   
   While testing, I noticed a possible small follow-up improvement in HASH
   overflow handling. Currently, any free overflow page may be reused, which
   can scatter overflow chains and hurt cache locality. Reusing recently freed
   overflow pages first could help, without changing WAL behavior or on-disk
   format.
   
   I would like to work on this as a follow-up enhancement and would welcome
   any suggestions.
   
   Best regards,
   Lakshmi
   
   On Tue, Dec 23, 2025 at 2:31 PM Kirill Reshke <reshkekirill@gmail.com>
   wrote:
   
   > There exists an optimization to index creation process, when we omit
   > to write any WAL
   > for index build. It is currently supported in B Tree, GIN, GiST, spg
   > indexes.
   > It works because we do not need to recover anything if index creation
   > fails, because if was not used by any query. So, the index can be
   > built on-disk, and then, just before making the index alive, we can
   > simply log all pages to WAL.
   >
   > Hash index currently lacks this optimization.
   > PFA implementation.
   >
   > During my testing, I checked the amount of WAL generated by index
   > build before and after patch applied. My script was something like:
   >
   > select pg_current_wal_insert_lsn();
   >
   > create index on t using hash (i);
   >
   > select pg_current_wal_insert_lsn();
   >
   > select pg_lsn_wal_diff(lsn1, lsn2);
   >
   > Resulting numbers depend on index size, but I got 2.5-3.5 times less
   > WAL with this patch and 8 times less WAL with this patch +
   > wal_compression=on.
   > Index creation time, however, did not change much...
   >
   > About implementation:
   > These are many types of record that can be generated during index build.
   > I know for sure these are possible (double-checked using pg_waldump):
   >
   > SPLIT_COMPLETE
   > INSERT
   > SPLIT_ALLOCATE_PAGE
   > SPLIT_PAGE
   > ADD_OVFL_PAGE
   > SQUEEZE_PAGE
   > INIT_META_PAGE
   > INIT_BITMAP_PAGE
   >
   >
   > Looks like  SPLIT_COMPLETE and VACUUM_ONE_PAGE are never generated
   > during index build. I'm not sure about  MOVE_PAGE_CONTENTS.
   >
   > So, implementation is simply pass isbuild flag everywhere something is
   > wal-logged. Looks like it is less invasive than alternatives.
   >
   > --
   > Best regards,
   > Kirill Reshke
   >