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* Strange avg value size for index on expression in pg_stats
@ 2014-11-10 10:52 Jehan-Guillaume de Rorthais <ioguix@free.fr>
2014-11-10 11:00 ` Re: Strange avg value size for index on expression in pg_stats Jehan-Guillaume de Rorthais <ioguix@free.fr>
2014-11-10 14:39 ` Re: Strange avg value size for index on expression in pg_stats Tom Lane <tgl@sss.pgh.pa.us>
0 siblings, 2 replies; 4+ messages in thread
From: Jehan-Guillaume de Rorthais @ 2014-11-10 10:52 UTC (permalink / raw)
To: pgsql-general
Hello,
I'm not sure if I should post here or on pgsql-hackers.
While investigating about a wrong result with my btree bloat estimation query,
I found a strange stat deviation between the average size of a value in its
table and its average size in one index on the "lower" expression.
Take the following scenario:
postgres@test=# create table test as
test-# select md5(t::text) AS t
test-# from generate_series(1, 1000000) t;
SELECT 1000000
postgres@test=# create index ON test (lower(t));
CREATE INDEX
postgres@test=# analyze test;
ANALYZE
postgres@test=# select tablename, attname, avg_width
test-# from pg_stats
test-# where schemaname = 'public';
tablename | attname | avg_width
----------------+---------+-----------
test | t | 33
test_lower_idx | lower | 36
Md5 values are always 32 bytes long. In the table, pg_stats reports 33 because
of the text header. In the index, the reported value is 36!
Looking at the page layout documentation and in the index using hexdump, I can
not find any answer about this 3 bytes. PFA the "hexdump -C" output from the
index. For each row, we clearly see a 8 bytes row header followed by a ONE byte
value header (43 'C'), the 32 bytes of the md5 and 7 bytes of padding (00).
A wild guess would be that ANALYZE is considering a text field from an
expression has always a 4 bytes header whatever its actual size (text field
header size is one if the value is < 127 bytes long, 4 in other situations).
This tiny difference is the source of a very bad estimation with the Btree bloat
estimation query when values are around an alignement boundary. As instance,
here is the use case that lead me to this:
tablename | attname | avg_width
--------------------+---------+-----------
customer | email | 23
customer_lower_idx | lower | 26
We have an index on email, and another one on lower(index). The first one is
aligned on 24, the second one on 32. Leading to bloat estimation of 17% for
the first one and -1% for the second one (the estimated index size is bigger
than the real one).
Any information about this from a hacker? Do anyone have an explanation about
this? Is it something that's worth posting on pgsql-hackers?
Regards,
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Attachments:
index-lower-hexdump-C.txt (text/plain, 25.8K) download
00000000 00 00 00 00 00 00 00 00 00 00 00 00 30 00 f0 1f |............0...|
00000010 f0 1f 04 20 00 00 00 00 62 31 05 00 02 00 00 00 |... ....b1......|
00000020 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 |................|
00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00001ff0 00 00 00 00 00 00 00 00 00 00 00 00 08 00 00 00 |................|
00002000 00 00 00 00 00 00 00 00 00 00 00 00 a8 01 30 0d |..............0.|
00002010 f0 1f 04 20 00 00 00 00 c0 9f 60 00 90 9f 60 00 |... ......`...`.|
00002020 60 9f 60 00 30 9f 60 00 00 9f 60 00 d0 9e 60 00 |`.`.0.`...`...`.|
00002030 a0 9e 60 00 70 9e 60 00 40 9e 60 00 10 9e 60 00 |..`.p.`.@.`...`.|
00002040 e0 9d 60 00 b0 9d 60 00 80 9d 60 00 50 9d 60 00 |..`...`...`.P.`.|
00002050 20 9d 60 00 f0 9c 60 00 c0 9c 60 00 90 9c 60 00 | .`...`...`...`.|
00002060 60 9c 60 00 30 9c 60 00 00 9c 60 00 d0 9b 60 00 |`.`.0.`...`...`.|
00002070 a0 9b 60 00 70 9b 60 00 40 9b 60 00 10 9b 60 00 |..`.p.`.@.`...`.|
00002080 e0 9a 60 00 b0 9a 60 00 80 9a 60 00 50 9a 60 00 |..`...`...`.P.`.|
00002090 20 9a 60 00 f0 99 60 00 c0 99 60 00 90 99 60 00 | .`...`...`...`.|
000020a0 60 99 60 00 30 99 60 00 00 99 60 00 d0 98 60 00 |`.`.0.`...`...`.|
000020b0 a0 98 60 00 70 98 60 00 40 98 60 00 10 98 60 00 |..`.p.`.@.`...`.|
000020c0 e0 97 60 00 b0 97 60 00 80 97 60 00 50 97 60 00 |..`...`...`.P.`.|
000020d0 20 97 60 00 f0 96 60 00 c0 96 60 00 90 96 60 00 | .`...`...`...`.|
000020e0 60 96 60 00 30 96 60 00 00 96 60 00 d0 95 60 00 |`.`.0.`...`...`.|
000020f0 a0 95 60 00 70 95 60 00 40 95 60 00 10 95 60 00 |..`.p.`.@.`...`.|
00002100 e0 94 60 00 b0 94 60 00 80 94 60 00 50 94 60 00 |..`...`...`.P.`.|
00002110 20 94 60 00 f0 93 60 00 c0 93 60 00 90 93 60 00 | .`...`...`...`.|
00002120 60 93 60 00 30 93 60 00 00 93 60 00 d0 92 60 00 |`.`.0.`...`...`.|
00002130 a0 92 60 00 70 92 60 00 40 92 60 00 10 92 60 00 |..`.p.`.@.`...`.|
00002140 e0 91 60 00 b0 91 60 00 80 91 60 00 50 91 60 00 |..`...`...`.P.`.|
00002150 20 91 60 00 f0 90 60 00 c0 90 60 00 90 90 60 00 | .`...`...`...`.|
00002160 60 90 60 00 30 90 60 00 00 90 60 00 d0 8f 60 00 |`.`.0.`...`...`.|
00002170 a0 8f 60 00 70 8f 60 00 40 8f 60 00 10 8f 60 00 |..`.p.`.@.`...`.|
00002180 e0 8e 60 00 b0 8e 60 00 80 8e 60 00 50 8e 60 00 |..`...`...`.P.`.|
00002190 20 8e 60 00 f0 8d 60 00 c0 8d 60 00 90 8d 60 00 | .`...`...`...`.|
000021a0 60 8d 60 00 30 8d 60 00 30 8d 60 00 00 00 00 00 |`.`.0.`.0.`.....|
000021b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00002d30 00 00 00 00 53 00 30 40 43 66 65 39 66 63 32 38 |....S.0@Cfe9fc28|
00002d40 39 63 33 66 66 30 61 66 31 34 32 62 36 64 33 62 |9c3ff0af142b6d3b|
00002d50 65 61 64 39 38 61 39 32 33 00 00 00 00 00 00 00 |ead98a923.......|
00002d60 00 00 00 00 41 00 30 40 43 66 63 34 39 30 63 61 |....A.0@Cfc490ca|
00002d70 34 35 63 30 30 62 31 32 34 39 62 62 65 33 35 35 |45c00b1249bbe355|
00002d80 34 61 34 66 64 66 36 66 62 00 00 00 00 00 00 00 |4a4fdf6fb.......|
00002d90 00 00 00 00 4c 00 30 40 43 66 62 64 37 39 33 39 |....L.0@Cfbd7939|
00002da0 64 36 37 34 39 39 37 63 64 62 34 36 39 32 64 33 |d674997cdb4692d3|
00002db0 34 64 65 38 36 33 33 63 34 00 00 00 00 00 00 00 |4de8633c4.......|
00002dc0 00 00 00 00 64 00 30 40 43 66 38 39 39 31 33 39 |....d.0@Cf899139|
00002dd0 64 66 35 65 31 30 35 39 33 39 36 34 33 31 34 31 |df5e105939643141|
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00002e00 33 63 38 33 33 64 66 66 34 62 33 38 66 63 38 64 |3c833dff4b38fc8d|
00002e10 32 38 37 32 66 31 65 63 36 00 00 00 00 00 00 00 |2872f1ec6.......|
00002e20 00 00 00 00 5e 00 30 40 43 66 34 62 39 65 63 33 |....^.0@Cf4b9ec3|
00002e30 30 61 64 39 66 36 38 66 38 39 62 32 39 36 33 39 |0ad9f68f89b29639|
00002e40 37 38 36 63 62 36 32 65 66 00 00 00 00 00 00 00 |786cb62ef.......|
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00002e70 37 31 34 35 61 37 32 63 30 00 00 00 00 00 00 00 |7145a72c0.......|
00002e80 00 00 00 00 50 00 30 40 43 66 30 33 33 61 62 33 |....P.0@Cf033ab3|
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00002ea0 39 64 30 33 37 30 32 38 64 00 00 00 00 00 00 00 |9d037028d.......|
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00002ec0 31 39 39 31 65 33 62 65 66 35 65 30 36 39 37 31 |1991e3bef5e06971|
00002ed0 33 61 66 39 66 61 36 63 61 00 00 00 00 00 00 00 |3af9fa6ca.......|
00002ee0 00 00 00 00 03 00 30 40 43 65 63 63 62 63 38 37 |......0@Ceccbc87|
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00002f20 63 34 36 30 38 32 33 32 65 30 37 64 33 61 61 33 |c4608232e07d3aa3|
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00003080 32 37 38 37 39 31 33 33 63 00 00 00 00 00 00 00 |27879133c.......|
00003090 00 00 00 00 28 00 30 40 43 64 36 34 35 39 32 30 |....(.0@Cd645920|
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000030d0 38 30 32 61 34 34 32 35 39 37 35 35 64 33 38 65 |802a44259755d38e|
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00003120 00 00 00 00 4f 00 30 40 43 64 31 66 65 31 37 33 |....O.0@Cd1fe173|
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00003340 32 66 38 65 64 35 64 32 31 61 34 34 62 30 65 35 |2f8ed5d21a44b0e5|
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00003380 35 31 63 39 62 64 65 31 31 00 00 00 00 00 00 00 |51c9bde11.......|
00003390 00 00 00 00 4a 00 30 40 43 61 64 36 31 61 62 31 |....J.0@Cad61ab1|
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000033f0 00 00 00 00 0e 00 30 40 43 61 61 62 33 32 33 38 |......0@Caab3238|
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000037e0 00 00 00 00 59 00 30 40 43 37 36 34 37 39 36 36 |....Y.0@C7647966|
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00003800 32 65 34 39 30 66 37 33 36 00 00 00 00 00 00 00 |2e490f736.......|
00003810 00 00 00 00 43 00 30 40 43 37 33 35 62 39 30 62 |....C.0@C735b90b|
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00003840 00 00 00 00 39 00 30 40 43 37 32 62 33 32 61 31 |....9.0@C72b32a1|
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00003870 00 00 00 00 11 00 30 40 43 37 30 65 66 64 66 32 |......0@C70efdf2|
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00004000
* Re: Strange avg value size for index on expression in pg_stats
2014-11-10 10:52 Strange avg value size for index on expression in pg_stats Jehan-Guillaume de Rorthais <ioguix@free.fr>
@ 2014-11-10 11:00 ` Jehan-Guillaume de Rorthais <ioguix@free.fr>
1 sibling, 0 replies; 4+ messages in thread
From: Jehan-Guillaume de Rorthais @ 2014-11-10 11:00 UTC (permalink / raw)
To: pgsql-general
On Mon, 10 Nov 2014 11:52:54 +0100
Jehan-Guillaume de Rorthais <ioguix@free.fr> wrote:
> Hello,
>
> I'm not sure if I should post here or on pgsql-hackers.
>
> While investigating about a wrong result with my btree bloat estimation query,
> I found a strange stat deviation between the average size of a value in its
> table and its average size in one index on the "lower" expression.
Oh, I forgot to add links to this query, for testing purpose:
* https://gist.github.com/ioguix/c29d5790b8b93bf81c27
* explanations, see http://blog.ioguix.net/tag/bloat/
Cheers,
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^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Strange avg value size for index on expression in pg_stats
2014-11-10 10:52 Strange avg value size for index on expression in pg_stats Jehan-Guillaume de Rorthais <ioguix@free.fr>
@ 2014-11-10 14:39 ` Tom Lane <tgl@sss.pgh.pa.us>
2014-11-10 15:41 ` Re: Strange avg value size for index on expression in pg_stats Jehan-Guillaume de Rorthais <ioguix@free.fr>
1 sibling, 1 reply; 4+ messages in thread
From: Tom Lane @ 2014-11-10 14:39 UTC (permalink / raw)
To: Jehan-Guillaume de Rorthais <ioguix@free.fr>; +Cc: pgsql-general
Jehan-Guillaume de Rorthais <ioguix@free.fr> writes:
> While investigating about a wrong result with my btree bloat estimation query,
> I found a strange stat deviation between the average size of a value in its
> table and its average size in one index on the "lower" expression.
ANALYZE doesn't look at the physical index at all. For an expression
index, it will recompute the index expression at each sampled row, and
then take statistics on those values. In the case you've got here, it's
re-running the lower() function and looking at the uncompressed result.
So that accounts for the discrepancy in average width stats.
> This tiny difference is the source of a very bad estimation with the
> Btree bloat estimation query when values are around an alignement
> boundary.
TBH, if that query is relying on ANALYZE width estimates to be accurate
to the last byte, its reliability is going to be horrid anyway.
regards, tom lane
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^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Strange avg value size for index on expression in pg_stats
2014-11-10 10:52 Strange avg value size for index on expression in pg_stats Jehan-Guillaume de Rorthais <ioguix@free.fr>
2014-11-10 14:39 ` Re: Strange avg value size for index on expression in pg_stats Tom Lane <tgl@sss.pgh.pa.us>
@ 2014-11-10 15:41 ` Jehan-Guillaume de Rorthais <ioguix@free.fr>
0 siblings, 0 replies; 4+ messages in thread
From: Jehan-Guillaume de Rorthais @ 2014-11-10 15:41 UTC (permalink / raw)
To: Tom Lane <tgl@sss.pgh.pa.us>; +Cc: pgsql-general
On Mon, 10 Nov 2014 09:39:23 -0500
Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Jehan-Guillaume de Rorthais <ioguix@free.fr> writes:
> > While investigating about a wrong result with my btree bloat estimation
> > query, I found a strange stat deviation between the average size of a value
> > in its table and its average size in one index on the "lower" expression.
>
> ANALYZE doesn't look at the physical index at all. For an expression
> index, it will recompute the index expression at each sampled row, and
> then take statistics on those values. In the case you've got here, it's
> re-running the lower() function and looking at the uncompressed result.
> So that accounts for the discrepancy in average width stats.
Ok, understood.
But in my demo scenario, I used *only* md5 to populate the test table. So
data length is always 32 and the average width length is exact. No possible
deviation, even with lower(). To be quite dense:
postgres@test=# select
length(lower(md5('a'))) = length(md5('b')),
length(md5('c')) = length(md5('d'));
-[ RECORD 1 ]
?column? | t
?column? | t
And here is another test with a static string for all rows:
postgres@test=# create table test as
select '1234567890123456789012'::text as t
from generate_series(1,100);
SELECT 100
postgres@test=# create index ON test (lower(t));
CREATE INDEX
postgres@test=# analyze test;
ANALYZE
postgres@test=# select tablename, attname, avg_width
from pg_stats
where schemaname = 'public';
tablename | attname | avg_width
----------------+---------+-----------
test | t | 23
test_lower_idx | lower | 26
AFAIU, we should not have a discrepancy here.
> > This tiny difference is the source of a very bad estimation with the
> > Btree bloat estimation query when values are around an alignement
> > boundary.
>
> TBH, if that query is relying on ANALYZE width estimates to be accurate
> to the last byte, its reliability is going to be horrid anyway.
Well, I'm aware of that. I don't need an accuracy to the last byte. This query
doesn't even pay attention to the data alignment padding anyway (I measured some
10% deviation in a case because of this).
This request only helps guessing the bloat evolution in Btrees or quickly
discover *big* deviations. In many situations, we can not afford a call to
pgstattuple.avg_leaf_density().
But this statistic difference between two values with the exact same size is
itching me. Sa far, I couldn't find a logical explanation and it just looks
like a wrong statistic.
Regards,
--
Jehan-Guillaume (ioguix) de Rorthais
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^ permalink raw reply [nested|flat] 4+ messages in thread
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Thread overview: 4+ messages (download: mbox mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
2014-11-10 10:52 Strange avg value size for index on expression in pg_stats Jehan-Guillaume de Rorthais <ioguix@free.fr>
2014-11-10 11:00 ` Jehan-Guillaume de Rorthais <ioguix@free.fr>
2014-11-10 14:39 ` Tom Lane <tgl@sss.pgh.pa.us>
2014-11-10 15:41 ` Jehan-Guillaume de Rorthais <ioguix@free.fr>
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