When building statistics, we need to decide how many rows to sample and
how accurate the resulting statistics should be. Until now, it was not
possible to explicitly define statistics target for extended statistics
objects, the value was always computed from the per-attribute targets
with a fallback to the system-wide default statistics target.
That's a bit inconvenient, as it ties together the statistics target set
for per-column and extended statistics. In some cases it may be useful
to require larger sample / higher accuracy for extended statics (or the
other way around), but with this approach that's not possible.
So this commit introduces a new command, allowing to specify statistics
target for individual extended statistics objects, overriding the value
derived from per-attribute targets (and the system default).
ALTER STATISTICS stat_name SET STATISTICS target_value;
When determining statistics target for an extended statistics object we
first look at this explicitly set value. When this value is -1, we fall
back to the old formula, looking at the per-attribute targets first and
then the system default. This means the behavior is backwards compatible
with older PostgreSQL releases.
Author: Tomas Vondra
Discussion: https://postgr.es/m/20190618213357.vli3i23vpkset2xd@development
Reviewed-by: Kirk Jamison, Dean Rasheed
For most uses of acl.h the details of how "Acl" internally looks like
are irrelevant. It might make sense to move a lot of the
implementation details into a separate header at a later point.
The main motivation of this change is to avoid including fmgr.h (via
array.h, which needs it for exposed structs) in a lot of files that
otherwise don't need it. A subsequent commit will remove the fmgr.h
include from a lot of files.
Directly include utils/array.h and utils/expandeddatum.h from the
files that need them, but previously included them indirectly, via
acl.h.
Author: Andres Freund
Discussion: https://postgr.es/m/20190803193733.g3l3x3o42uv4qj7l@alap3.anarazel.de
The current extended statistics code was a bit confused which collation
to use. When building the statistics, the collations defined as default
for the data types were used (since commit 5e0928005). The MCV code was
however using the column collations for MCV serialization, and then
DEFAULT_COLLATION_OID when computing estimates. So overall the code was
using all three possible options, inconsistently.
This uses the column colation everywhere - this makes it consistent with
what 5e0928005 did for regular stats. We however do not track the
collations in a catalog, because we can derive them from column-level
information. This may need to change in the future, e.g. after allowing
statistics on expressions.
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/8736jdhbhc.fsf%40ansel.ydns.eu
Backpatch-to: 12
The examine_opclause_expression function needs to return information on
which side of the operator we found the Var, but the variable was called
"isgt" which is rather misleading (it assumes the operator is either
less-than or greater-than, but it may be equality or something else).
Other places in the planner use a variable called "varonleft" for this
purpose, so just adopt the same convention here.
The code also assumed we don't care about this flag for equality, as
(Var = Const) and (Const = Var) should be the same thing. But that does
not work for cross-type operators, in which case we need to pass the
parameters to the procedure in the right order. So just use the same
code for all types of expressions.
This means we don't need to care about the selectivity estimation
function anymore, at least not in this code. We should only get the
supported cases here (thanks to statext_is_compatible_clause).
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/8736jdhbhc.fsf%40ansel.ydns.eu
Backpatch-to: 12
When evaluating clauses on a multivariate MCV list, we build a bitmap
tracking how the clauses match each item of the MCV list. When updating
the bitmap we need to consider the current value (tracking how the item
matches preceding clauses), match for the current clause and whether the
clauses are connected by AND or OR.
Until now the logic was copied on every place updating the bitmap, which
was not quite readable. So just move it to a separate function and call
it where needed.
Backpatch to 12, where the code was introduced. While not a bugfix, this
should make maintenance and future backpatches easier.
Discussion: https://postgr.es/m/8736jdhbhc.fsf%40ansel.ydns.eu
There were two issues in how the extended statistics handled NULL values
in opclauses. Firstly, the code was oblivious to the possibility that
Const may be NULL (constisnull=true) in which case the constvalue is
undefined. We need to treat this as a mismatch, and not call the proc.
Secondly, the MCV item itself may contain NULL values too - the code
already did check that, and updated the match bitmap accordingly, but
failed to ensure we won't call the operator procedure anyway. It did
work for AND-clauses, because in that case false in the bitmap stops
evaluation of further clauses. But for OR-clauses ir was not easy to
get incorrect estimates or even trigger a crash.
This fixes both issues by extending the existing check so that it looks
at constisnull too, and making sure it skips calling the procedure.
Discussion: https://postgr.es/m/8736jdhbhc.fsf%40ansel.ydns.eu
We expect opclauses to have exactly one Var and one Const, but the code
was checking the Const by calling is_pseudo_constant_clause() which is
incorrect - we need a proper constant.
Fixed by using plain IsA(x,Const) to check type of the node. We need to
do these checks in two places, so move it into a separate function that
can be called in both places.
Reported by Andreas Seltenreich, based on crash reported by sqlsmith.
Backpatch to v12, where this code was introduced.
Discussion: https://postgr.es/m/8736jdhbhc.fsf%40ansel.ydns.eu
Backpatch-to: 12
The TYPECACHE_GT_OPR is not needed (it used to be in older version of
the MCV code), but the compiler failed to detect this as the result was
used in a fmgr_info() call, populating a FmgrInfo entry.
Backpatch to v12, where this code was introduced.
Discussion: https://postgr.es/m/8736jdhbhc.fsf%40ansel.ydns.eu
Backpatch-to: 12
The itemlen variable used to be referenced in multiple places, but since
reworking the serialization code it's used only in one assert. Fixed by
removing the variable and calling the macro from the assert directly.
Backpatch to 12, where this code was introduced.
Reported-by: Jeff Janes
Discussion: https://postgr.es/m/CAMkU=1zc_ovH9NZd_9ovuiEWkF9yX06URUDdXCmgDydf-bqB5A@mail.gmail.com
The serialization format of multivariate MCV lists included alignment in
order to allow direct access to part of the serialized data, but despite
multiple fixes (see for example commits d85e0f366a and ea4e1c0e8f) this
proved to be problematic.
This commit abandons alignment in the serialized format, and just copies
everything during deserialization. We now also track amount of memory
needed after deserialization (including alignment), which allows us to
deserialize the MCV list in a single pass.
Bump catversion, as this affects contents of pg_statistic_ext_data.
Backpatch to 12, where multi-column MCV lists were introduced.
Author: Tomas Vondra
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/2201.1561521148@sss.pgh.pa.us
The function pg_mcv_list_items() returns values stored in MCV items. The
items may contain columns with different data types, so the function was
generating text array-like representation, but in an ad-hoc way without
properly escaping various characters etc.
Fixed by simply building a text[] array, which also makes it easier to
use from queries etc.
Requires changes to pg_proc entry, so bump catversion.
Backpatch to 12, where multi-column MCV lists were introduced.
Author: Tomas Vondra
Reviewed-by: Dean Rasheed
Discussion: https://postgr.es/m/20190618205920.qtlzcu73whfpfqne@development
When building multi-column MCV lists, we compute base frequency for each
item, i.e. a product of per-column frequencies for values from the item.
As a value may be in multiple groups, the code was scanning the whole
array of groups while adding items to the MCV list. This works fine as
long as the number of distinct groups is small, but it's easy to trigger
trigger O(N^2) behavior, especially after increasing statistics target.
This commit precomputes frequencies for values in all columns, so that
when computing the base frequency it's enough to make a simple bsearch
lookup in the array.
Backpatch to 12, where multi-column MCV lists were introduced.
Discussion: https://postgr.es/m/20190618205920.qtlzcu73whfpfqne@development
Since extended statistic got introduced in PostgreSQL 10, there was a
single catalog pg_statistic_ext storing both the definitions and built
statistic. That's however problematic when a user is supposed to have
access only to the definitions, but not to user data.
Consider for example pg_dump on a database with RLS enabled - if the
pg_statistic_ext catalog respects RLS (which it should, if it contains
user data), pg_dump would not see any records and the result would not
define any extended statistics. That would be a surprising behavior.
Until now this was not a pressing issue, because the existing types of
extended statistic (functional dependencies and ndistinct coefficients)
do not include any user data directly. This changed with introduction
of MCV lists, which do include most common combinations of values.
The easiest way to fix this is to split the pg_statistic_ext catalog
into two - one for definitions, one for the built statistic values.
The new catalog is called pg_statistic_ext_data, and we're maintaining
a 1:1 relationship with the old catalog - either there are matching
records in both catalogs, or neither of them.
Bumped CATVERSION due to changing system catalog definitions.
Author: Dean Rasheed, with improvements by me
Reviewed-by: Dean Rasheed, John Naylor
Discussion: https://postgr.es/m/CAEZATCUhT9rt7Ui%3DVdx4N%3D%3DVV5XOK5dsXfnGgVOz_JhAicB%3DZA%40mail.gmail.com
This is still using the 2.0 version of pg_bsd_indent.
I thought it would be good to commit this separately,
so as to document the differences between 2.0 and 2.1 behavior.
Discussion: https://postgr.es/m/16296.1558103386@sss.pgh.pa.us
The memcpy() was copying type OIDs in the wrong direction, so the
deserialized MCV list always had them as 0. This is mostly harmless
except when printing the data in pg_mcv_list_items(), in which case
it reported
ERROR: cache lookup failed for type 0
Also added a simple regression test for pg_mcv_list_items() function,
printing a single-item MCV list.
Reported-By: Dean Rasheed
Discussion: https://postgr.es/m/CAEZATCX6T0iDTTZrqyec4Cd6b4yuL7euu4=rQRXaVBAVrUi1Cg@mail.gmail.com
Commit ea4e1c0e8f resolved issues with memory alignment in serialized
pg_mcv_list values, but it required copying data to/from the varlena
buffer during serialization and deserialization. As the MCV lits may
be fairly large, the overhead (memory consumption, CPU usage) can get
rather significant too.
This change tweaks the serialization format so that the alignment is
correct with respect to the varlena value, and so the parts may be
accessed directly without copying the data.
Catversion bump, as it affects existing pg_statistic_ext data.
Commit d85e0f366a tried to fix memory alignment issues in serialization
and deserialization of pg_mcv_list values, but it was a few bricks shy.
The arrays of uint16 indexes in serialized items was not aligned, and
the both the values and isnull flags were using the same pointer.
Per investigation by Tom Lane on gaur.
Blind attempt at fixing ia64, hppa an sparc builds.
The serialized representation of MCV lists did not enforce proper memory
alignment for internal fields, resulting in deserialization issues on
platforms that are more sensitive to this (ia64, sparc and hppa).
This forces a catalog version bump, because the layout of serialized
pg_mcv_list changes.
Broken since 7300a699.
There were multiple issues in deserialization of pg_mcv_list values.
Firstly, the data is loaded from syscache, but the deserialization was
performed after ReleaseSysCache(), at which point the data might have
already disappeared. Fixed by moving the calls in statext_mcv_load,
and using the same NULL-handling code as existing stats.
Secondly, the deserialized representation used pointers into the
serialized representation. But that is also unsafe, because the data
may disappear at any time. Fixed by reworking and simplifying the
deserialization code to always copy all the data.
And thirdly, when deserializing values for types passed by value, the
code simply did memcpy(d,s,typlen) which however does not work on
bigendian machines. Fixed by using fetch_att/store_att_byval.
The MCV build should always call get_mincount_for_mcv_list(), as the
there is no other logic to decide whether the MCV list represents all
the data. So just remove the (ngroups > nitems) condition.
Also, when building MCV lists, the number of items was limited by the
statistics target (i.e. up to 10000). But when deserializing the MCV
list, a different value (8192) was used to check the input, causing
an error. Simply ensure that the same value is used in both places.
This should have been included in 7300a69950, but I forgot to include it
in that commit.
Introduce a third extended statistic type, supported by the CREATE
STATISTICS command - MCV lists, a generalization of the statistic
already built and used for individual columns.
Compared to the already supported types (n-distinct coefficients and
functional dependencies), MCV lists are more complex, include column
values and allow estimation of much wider range of common clauses
(equality and inequality conditions, IS NULL, IS NOT NULL etc.).
Similarly to the other types, a new pseudo-type (pg_mcv_list) is used.
Author: Tomas Vondra
Reviewed-by: Dean Rasheed, David Rowley, Mark Dilger, Alvaro Herrera
Discussion: https://postgr.es/m/dfdac334-9cf2-2597-fb27-f0fb3753f435@2ndquadrant.com
