[code notes] the implementation of alter table
Overview
In this article, I will inspect the postgresql code to find out the implementation of alter table
command, specifically, the add column subcommand of the alter table
command. The code for this article is from postgresql commit hash 21e3a8bc3544a1cfcff85933bc9c0664af32a8b8.
usage
alter table myt add col1 int;
alter table myt alter col2 type text;
alter table myt add constraint col2_min_length CHECK (lenght(col2) > 6);
alter table myt set unlogged;
syntax
![[Pasted image 20240323143948.png]]
![[Pasted image 20240323144141.png]]
Each alter table
command has a AlterTableStmt
data structure. An AlterTableStmt
includes one or more AlterTableCmd
commands. subcommands
are add
, drop
, rename
, set
, etc.
execution
code path
exec_simple_query
PortalRun
ProcessUtility
ProcessUtilitySlow
AlterTable
ATController
ATRewriteCatalogs
ATExecCmd
ATExecAddColumn
ATParseTransformCmd
transformAlterTableStmt
Inside function ProcessUtilitySlow
, before function AlterTable
, a function AlterTableGetLockLevel
is called to lock the being-altered table.
transformAlterTableStmt
transformAlterTableStmt
transform original AlterTableCmd
commands into more new AlterTableCmd
commands.
- open the being-altered table without any lock
- set up pstate
- set up
CreateStmtContext
- for sql
alter table add col1 int
, it needs invoketransformColumnDefinition
which invokestransformColumnType
. For this simple sql,transformColumnType
only verifies the existence of the column type. - Postprocess constraints, include index constraints, foreign key constraints, check constraints
- if any constraint exists, create a new
AlterTableCmd
with typeAT_AddConstraint
. - close the table
- record the new list of
AlterTableCmd
commands inAlterTableStmt
ATExecAddColumn
- since this function recurses, it could be driven to stack overflow, hence
check_stack_depth
now. - At top level, permission check was done in ATPrepCmd, else do it
- open
pg_attribute
table withRowExclusiveLock
- check for column name collision
- do parse transformation through
ATParseTransformCmd
- update information in
pg_attribute
table, throughInsertPgAttributeTuples
- update information in
pg_class
table, throughCatalogTupleUpdate
- run post creation hook for new attribute
- Make the attribute's catalog entry visible, through
CommandCounterIncrement
- Store the DEFAULT, if any, in the catalogs
- Add needed dependency entries for the new column.
- find any inherited tables by inspecting
pg_class.relhassubclass
- if found, then recursively invokes
ATExecAddColumn
- returns the
ObjectAddress
which represents the newly added column
posted on 2024-03-23 18:37 winter-loo 阅读(5) 评论(0) 编辑 收藏 举报