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| author | Tom Lane <tgl@sss.pgh.pa.us> | 2020-08-30 12:21:51 -0400 |
|---|---|---|
| committer | Tom Lane <tgl@sss.pgh.pa.us> | 2020-08-30 12:21:51 -0400 |
| commit | 3d351d916b20534f973eda760cde17d96545d4c4 (patch) | |
| tree | 133283e6cd7c7308add384bd7fd5d037801ac683 /doc/src | |
| parent | 9511fb37ac78c77736e5483118265f7e83cd9f3c (diff) | |
| download | postgresql-3d351d916b20534f973eda760cde17d96545d4c4.tar.gz postgresql-3d351d916b20534f973eda760cde17d96545d4c4.zip | |
Redefine pg_class.reltuples to be -1 before the first VACUUM or ANALYZE.
Historically, we've considered the state with relpages and reltuples
both zero as indicating that we do not know the table's tuple density.
This is problematic because it's impossible to distinguish "never yet
vacuumed" from "vacuumed and seen to be empty". In particular, a user
cannot use VACUUM or ANALYZE to override the planner's normal heuristic
that an empty table should not be believed to be empty because it is
probably about to get populated. That heuristic is a good safety
measure, so I don't care to abandon it, but there should be a way to
override it if the table is indeed intended to stay empty.
Hence, represent the initial state of ignorance by setting reltuples
to -1 (relpages is still set to zero), and apply the minimum-ten-pages
heuristic only when reltuples is still -1. If the table is empty,
VACUUM or ANALYZE (but not CREATE INDEX) will override that to
reltuples = relpages = 0, and then we'll plan on that basis.
This requires a bunch of fiddly little changes, but we can get rid of
some ugly kluges that were formerly needed to maintain the old definition.
One notable point is that FDWs' GetForeignRelSize methods will see
baserel->tuples = -1 when no ANALYZE has been done on the foreign table.
That seems like a net improvement, since those methods were formerly
also in the dark about what baserel->tuples = 0 really meant. Still,
it is an API change.
I bumped catversion because code predating this change would get confused
by seeing reltuples = -1.
Discussion: https://postgr.es/m/F02298E0-6EF4-49A1-BCB6-C484794D9ACC@thebuild.com
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/catalogs.sgml | 4 | ||||
| -rw-r--r-- | doc/src/sgml/fdwhandler.sgml | 3 |
2 files changed, 6 insertions, 1 deletions
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml index 9fe260ecff7..1d1b8ce8fb1 100644 --- a/doc/src/sgml/catalogs.sgml +++ b/doc/src/sgml/catalogs.sgml @@ -1977,6 +1977,10 @@ SCRAM-SHA-256$<replaceable><iteration count></replaceable>:<replaceable>&l the planner. It is updated by <command>VACUUM</command>, <command>ANALYZE</command>, and a few DDL commands such as <command>CREATE INDEX</command>. + If the table has never yet been vacuumed or + analyzed, <structfield>reltuples</structfield> + contains <literal>-1</literal> indicating that the row count is + unknown. </para></entry> </row> diff --git a/doc/src/sgml/fdwhandler.sgml b/doc/src/sgml/fdwhandler.sgml index 74793035d7f..72fa1272120 100644 --- a/doc/src/sgml/fdwhandler.sgml +++ b/doc/src/sgml/fdwhandler.sgml @@ -130,7 +130,8 @@ GetForeignRelSize(PlannerInfo *root, (The initial value is from <structname>pg_class</structname>.<structfield>reltuples</structfield> which represents the total row count seen by the - last <command>ANALYZE</command>.) + last <command>ANALYZE</command>; it will be <literal>-1</literal> if + no <command>ANALYZE</command> has been done on this foreign table.) </para> <para> |