dbDelta PRIMARY KEY Single Space Recreates Index

[charlestonsw]

Reference ticket #10404.

This is to decompose the original ticket into components. May be fixed in 4.4. Needs testing.

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If there is a double space after PRIMARY KEY the dbDelta method will create the primary key and future comparisons match, reducing overhead whenever dbDelta is called (on a plugin/theme update for example).

On a large table, hundreds of thousands of records, this can be a notable performance penalty when updating a plugin/theme.

This avoids performance penalties:

PRIMARY KEY  my_pkey ( id ),

This recreates the index:

PRIMARY KEY my_pkey ( id ),

[ocean90]

In 37583:

Database: Normalize index definitions in dbDelta().

dbDelta() compares the index definitions against the result of SHOW INDEX FROM $table_name. This requires a specific format so indices are not unnecessarily re-created. This format wasn't ensured, until now.

• Parse the raw index definition to extract the type, name and columns so a normalized definition can be built (#20263, #34873).
• Standardize on uppercase types (#34871) and on 'KEY'. 'INDEX' is only a synonym for 'KEY'.
• Escape index names with backticks (#20263).
• Normalize columns: Ignore ASC and DESC definitions (#34959), remove whitespaces (#34869) and escape column names with backticks (#20263).
• Add backticks to all index change queries (#20263).

Props ocean90, pento, kurtpayne.
Fixes #20263, #34869, #34871, #34873, #34959.