Re: WAL being written during SELECT * query

[Tom DalPozzo <t.dalpozzo@xxxxxxxxx>]

2017-04-06 17:51 GMT+02:00 Tom DalPozzo <t.dalpozzo@xxxxxxxxx>:

2017-04-04 19:18 GMT+02:00 Scott Marlowe <scott.marlowe@xxxxxxxxx>:

On Tue, Apr 4, 2017 at 9:46 AM, Tom DalPozzo <t.dalpozzo@xxxxxxxxx> wrote:
> Hi,
> I have a very big table (10GB).
> I noticed that many WAL segments are being written when elaborating read
> only transactions like this:
> select * from dati25600000000 where id >43000000 limit 1000000;
> I don't understand why are there WAL writings during read only transactions.
> Regards
> Pupillo



I think this is the db setting hint bits, but I'm no expert in that area.

Hi,

I'm not able to reproduce the problem in a deterministic way. Sometimes it does sometimes not.

Anyway, examining with pg_xlogdum those WAL, I get many many records like these:

rmgr: XLOG        len (rec/tot):      0/  8173, tx:          0, lsn: C/E502AF28, prev C/E5028F20, desc: FPI_FOR_HINT , blkref #0: rel 1663/16384/18846 blk 1454439 FPW

rmgr: XLOG        len (rec/tot):      0/  8173, tx:          0, lsn: C/E502CF30, prev C/E502AF28, desc: FPI_FOR_HINT , blkref #0: rel 1663/16384/18846 blk 1454440 FPW

rmgr: XLOG        len (rec/tot):      0/  8173, tx:          0, lsn: C/E502EF38, prev C/E502CF30, desc: FPI_FOR_HINT , blkref #0: rel 1663/16384/18846 blk 1454441 FPW

rmgr: XLOG        len (rec/tot):      0/  8173, tx:          0, lsn: C/E5030F40, prev C/E502EF38, desc: FPI_FOR_HINT , blkref #0: rel 1663/16384/18846 blk 1454442 FPW

rmgr: XLOG        len (rec/tot):      0/  8173, tx:          0, lsn: C/E5032F48, prev C/E5030F40, desc: FPI_FOR_HINT , blkref #0: rel 1663/16384/18846 blk 1454443 FPW

What is the meaning of FPI_FOR_HINT?

I've replication slots and async replication, but slaves are kept off (PCs not powered).

No other queries except that one.

Even select count (*) from dati25600000000; gives the problem (sometimes)
Thanks

Pupillo

Hi, I can reproduce the problem.

1) Insert many rows (>50 milions) in the table.

2) perform a select like select count(*) in order to force the system to read all the rows.

The FIRST time you do step 2) it writes all those FP_FOR_HINTS recs.

Next times or if the table is quite small, it doesn't do the problem.

If, instead of select count (*) you make a select which involved only a group of tuples, it write only the recs regarding those tuples. 

Regards

Pupillo