Re: Statistics Import and Export

Hi Corey,

On Sat, Mar 23, 2024 at 7:21 AM Corey Huinker <corey.huinker@gmail.com> wrote:

v12 attached.

0001 - 

Some random comments

+SELECT
+    format('SELECT pg_catalog.pg_set_attribute_stats( '
+            || 'relation => %L::regclass::oid, attname => %L::name, '
+            || 'inherited => %L::boolean, null_frac => %L::real, '
+            || 'avg_width => %L::integer, n_distinct => %L::real, '
+            || 'most_common_vals => %L::text, '
+            || 'most_common_freqs => %L::real[], '
+            || 'histogram_bounds => %L::text, '
+            || 'correlation => %L::real, '
+            || 'most_common_elems => %L::text, '
+            || 'most_common_elem_freqs => %L::real[], '
+            || 'elem_count_histogram => %L::real[], '
+            || 'range_length_histogram => %L::text, '
+            || 'range_empty_frac => %L::real, '
+            || 'range_bounds_histogram => %L::text) ',
+        'stats_export_import.' || s.tablename || '_clone', s.attname,
+        s.inherited, s.null_frac,
+        s.avg_width, s.n_distinct,
+        s.most_common_vals, s.most_common_freqs, s.histogram_bounds,
+        s.correlation, s.most_common_elems, s.most_common_elem_freqs,
+        s.elem_count_histogram, s.range_length_histogram,
+        s.range_empty_frac, s.range_bounds_histogram)
+FROM pg_catalog.pg_stats AS s
+WHERE s.schemaname = 'stats_export_import'
+AND s.tablename IN ('test', 'is_odd')
+\gexec

Why do we need to construct the command and execute? Can we instead execute the function directly? That would also avoid ECHO magic.

+   <table id="functions-admin-statsimport">
+    <title>Database Object Statistics Import Functions</title>
+    <tgroup cols="1">
+     <thead>
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        Function
+       </para>
+       <para>
+        Description
+       </para></entry>
+      </row>
+     </thead>

COMMENT: The functions throw many validation errors. Do we want to list the acceptable/unacceptable input values in the documentation corresponding to those? I don't expect one line per argument validation. Something like "these, these and these arguments can not be NULL" or "both arguments in each of the pairs x and y, a and b, and c and d should be non-NULL or NULL respectively".

+ /* Statistics are dependent on the definition, not the data */
+ /* Views don't have stats */
+ if ((tbinfo->dobj.dump & DUMP_COMPONENT_STATISTICS) &&
+ (tbinfo->relkind == RELKIND_VIEW))
+ dumpRelationStats(fout, &tbinfo->dobj, reltypename,
+  tbinfo->dobj.dumpId);
+

Statistics are about data. Whenever pg_dump dumps some filtered data, the
statistics collected for the whole table are uselss. We should avoide dumping
statistics in such a case. E.g. when only schema is dumped what good is
statistics? Similarly the statistics on a partitioned table may not be useful
if some its partitions are not dumped. Said that dumping statistics on foreign
table makes sense since they do not contain data but the statistics still makes sense.

Whether or not I pass --no-statistics, there is no difference in the dump output. Am I missing something?

$ pg_dump -d postgres > /tmp/dump_no_arguments.out
$ pg_dump -d postgres --no-statistics > /tmp/dump_no_statistics.out
$ diff /tmp/dump_no_arguments.out /tmp/dump_no_statistics.out

$

IIUC, pg_dump includes statistics by default. That means all our pg_dump related tests will have statistics output by default. That's good since the functionality will always be tested. 1. We need additional tests to ensure that the statistics is installed after restore. 2. Some of those tests compare dumps before and after restore. In case the statistics is changed because of auto-analyze happening post-restore, these tests will fail.