Parse MySQL column default value #110

billy1624 · 2023-06-07T08:42:34Z

PR Info

Closes Fail to distinguish MySQL default expression / value #23

Bug Fixes

Parse MySQL column default value

tyt2y3 · 2023-06-08T09:48:11Z

src/mysql/parser/column.rs

+                        default[1..(default.len() - 1)].into(),
+                    ))
+                } else if default == "NULL" {
+                    None


I think 'default NULL' has a different meaning to 'no default'.
Seems like we should add a new variant ColumnDefault::Null.

In practice, the default value could be 0 or '' instead of NULL.

tyt2y3 · 2023-06-08T10:13:16Z

src/mysql/parser/column.rs

@@ -264,7 +264,25 @@ pub fn parse_column_default(column_default: Option<String>) -> Option<ColumnDefa
    match column_default {
        Some(default) => {
            if !default.is_empty() {


I just read https://dev.mysql.com/doc/refman/8.0/en/data-type-defaults.html again,
I think the logic should be as follows:

parse the EXTRA column; if it contains DEFAULT_GENERATED, then the COLUMN_DEFAULT should be regarded as an expression

if the default is an expression, parse it as a Expr and recognize the keywords CURRENT_TIMESTAMP, otherwise Custom

if the default is not an expression, then it's a literal; then we should
a. if it is numeric-like then regard it as Number
b. otherwise regard it as String

1. if matches a keyword (CURRENT_TIMESTAMP) then make it a corresponding keyword

if it starts with a ( regard it as an expression

if it is numeric-like then regard it as Number. not sure whether it's worth the effort to classify into int and real/decimal

finally, consider it a String

It's really a legacy that MySQL does not quote a literal string. It's given as hello instead of 'hello'.

Edit: I was wrong; apparently COLUMNS table does not give a () for expressions either.

I think we cannot match the ( character. The expression (CURRENT_TIMESTAMP) or CURRENT_TIMESTAMP both are valid syntax to define default value on table creation. But when you query the default expression via information schema, you get different result.

billy1624 · 2023-06-08T12:26:26Z

parse the EXTRA column; if it contains DEFAULT_GENERATED, then the COLUMN_DEFAULT should be regarded as an expression

Hmmm... no. This logic doesn't work on every MySQL / Mariadb version

tyt2y3 · 2023-06-08T12:31:36Z

May be we simply regard CURRENT_TIMESTAMP as special, but only if the column type is timestamp or datatime.
Note that, CURRENT_DATE is not special.

Before MariaDB 10.2.1 you couldn't usually provide an expression or function to evaluate at insertion time. You had to provide a constant default value instead. The one exception is that you may use CURRENT_TIMESTAMP as the default value for a TIMESTAMP column to use the current timestamp at insertion time.
CURRENT_TIMESTAMP may also be used as the default value for a DATETIME
From MariaDB 10.2.1 you can use most functions in DEFAULT. Expressions should have parentheses around them

and this https://mariadb.com/kb/en/information-schema-columns-table/ https://jira.mariadb.org/browse/MDEV-13132
apparently MariaDB would sometimes quote the literal

Default value for the column. From MariaDB 10.2.7, literals are quoted to distinguish them from expressions. NULL means that the column has no default. In MariaDB 10.2.6 and earlier, no quotes were used for any type of default and NULL can either mean that there is no default, or that the default column value is NULL.

tyt2y3 · 2023-06-08T12:44:07Z

Sadly to implement this correctly we have to perform some version check:

MySQL 5:
Only literal is supported, so:

if column is TIMESTAMP or DATETIME and value is CURRENT_TIMESTAMP, regard it as DefaultExpr::CurrentTimestamp
if it is numeric-like then regard it as Number
otherwise regard it as String

MySQL 8:

parse the EXTRA column; if it contains DEFAULT_GENERATED, then the COLUMN_DEFAULT should be regarded as an expression
if the default is an expression, parse it as a Expr and recognize the keywords CURRENT_TIMESTAMP, otherwise Custom
if the default is not an expression, then it's a literal; then we should
a. if it is numeric-like then regard it as Number
b. otherwise regard it as String

MariaDB < 10.2.1:
Same as MySQL 5

MariaDB >= 10.2.1:

if it starts with a ' it must be a String
if it is numeric-like then it is a Number
otherwise it is an expression, and can be one of the keywords (CURRENT_TIMESTAMP)

billy1624 · 2023-06-08T14:14:00Z

Hey @tyt2y3, was a success. Please proofread the db specific parsing part for me :)

tyt2y3 · 2023-06-08T14:20:51Z

src/mysql/parser/column.rs

+    if is_date_time_col && default == "CURRENT_DATE" {
+        ColumnDefault::CurrentDate
+    } else if is_date_time_col && default == "CURRENT_TIME" {
+        ColumnDefault::CurrentTime


Looks pretty promising! My only question is, are you sure current_date/time is 'as special as' current_timestamp?

Judging by the docs, it only mentioned CURRENT_TIMESTAMP:

The exception is that, for TIMESTAMP and DATETIME columns, you can specify the CURRENT_TIMESTAMP function as the default, without enclosing parentheses. See Section 11.2.5, “Automatic Initialization and Updating for TIMESTAMP and DATETIME”.

It didn't say CURRENT_DATE can be used for DATE, etc.

This means, for example, that you cannot set the default for a date column to be the value of a function such as NOW() or CURRENT_DATE.

I belive it does not work under MySQL 5.x

I just double checked, yes, you're right. current_date/time cannot be used as default value / expression.

Removed CurrentDate and CurrentTime fca3ee5

tyt2y3 · 2023-06-08T14:27:09Z

src/mysql/parser/column.rs

+}
+
+pub fn parse_mysql_8_default(default: String, extra: &str) -> ColumnDefault {
+    let is_expression = extra == "DEFAULT_GENERATED";


The extra can have more string parts, not just DEFAULT_GENERATED, a contains() will be safer.
The right way would be to split the string by ' ' and then Vec::contains.

tyt2y3 · 2023-06-08T14:36:40Z

src/mysql/parser/column.rs

+    } else if let Ok(double) = default.parse::<f64>() {
+        ColumnDefault::Double(double)
+    } else {
+        ColumnDefault::String(default)


in this case the tail case should be split:

is_expression = true -> ColumnDefault::CustomExpr

is_expression = false -> ColumnDefault::String

tyt2y3 · 2023-06-08T14:39:09Z

src/mysql/parser/column.rs

+    } else if default == "NULL" {
+        ColumnDefault::Null
+    } else {
+        ColumnDefault::String(default)


I believe the final case is a CustomExpr;
https://mariadb.com/kb/en/create-table/#default-column-option

billy1624 · 2023-06-09T07:28:44Z

Done

tyt2y3 · 2023-06-10T17:42:42Z

I just checked on my machine, MySQL 8 default (NULL) is valid and is different from default null

tyt2y3 · 2023-06-10T17:48:46Z

Wait... I am re-reading this

From MariaDB 10.2.7, literals are quoted to distinguish them from expressions. NULL means that the column has no default

It means for the versions >= 10.2.1 and < 10.2.7 is a void, and there is no way to properly parse the default.

github-actions · 2023-07-20T14:50:56Z

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Parse MySQL column default value

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billy1624 self-assigned this Jun 7, 2023

billy1624 mentioned this pull request Jun 7, 2023

Fail to distinguish MySQL default expression / value #23

Closed

billy1624 added 4 commits June 7, 2023 17:02

Fixup

43ed184

clippy

be97301

Fix

8c73afa

clippy

190c8a3

billy1624 marked this pull request as ready for review June 7, 2023 10:40

billy1624 requested a review from tyt2y3 June 7, 2023 11:34

tyt2y3 reviewed Jun 8, 2023

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billy1624 added 2 commits June 8, 2023 19:45

Add ColumnDefault::Null

86f794b

Parse extra to determine if it's default expression

3f23b2e

billy1624 added 2 commits June 8, 2023 21:45

Customized parsing logic for MySQL and MariaDB column default

cc51c1d

Merge remote-tracking branch 'origin/master' into parse-col-default-val

6c1ea4b

billy1624 requested a review from tyt2y3 June 8, 2023 14:16

tyt2y3 reviewed Jun 8, 2023

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billy1624 added 6 commits June 9, 2023 15:08

Remove current date and time from ColumnDefault

fca3ee5

Add ColumnDefault::CustomExpr

880b70d

Simplify

51e6b40

Parse custom expression

146c081

fmt

b078079

Fix

f11b494

billy1624 requested a review from tyt2y3 June 9, 2023 07:28

tyt2y3 added 2 commits June 11, 2023 01:10

use i64

ae0d2d1

Tweaks

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tyt2y3 approved these changes Jun 10, 2023

View reviewed changes

MariaDB >= 10.2.7

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tyt2y3 merged commit 1cd22b5 into master Jun 10, 2023

tyt2y3 deleted the parse-col-default-val branch June 10, 2023 18:01

tyt2y3 added a commit that referenced this pull request Jun 10, 2023

Customized parsing logic for MySQL and MariaDB column default (#110)

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Parse MySQL column default value #110

Parse MySQL column default value #110

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Parse MySQL column default value #110

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