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This article focuses on "how to understand the Innodb DB_ROLL_PTR pointer in MySQL". Interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn how to understand the Innodb DB_ROLL_PTR pointer in MySQL.

I. introduction

We know that each record has the following distribution on the clustered index:

Rowid (primary key) + DB_TRX_ID+DB_ROLL_PTR+ other fields

In this format, DB_TRX_ID+DB_ROLL_PTR is stored as key information for consistent reading, where DB_TRX_ID occupies 6 bytes on storage and DB_ROLL_PTR occupies 7 bytes on storage. So how does DB_ROLL_PTR translate to undo? this is also a question asked by a friend.

Here is the definition of this part in trx0types.h

/ * * Row identifier (DB_ROW_ID, DATA_ROW_ID) * / typedef ib_id_t row_id_t;/** Transaction identifier (DB_TRX_ID, DATA_TRX_ID) * / typedef ib_id_t trx_id_t;/** Rollback pointer (DB_ROLL_PTR, DATA_ROLL_PTR) * / typedef ib_id_t roll_ptr_t

And ib_id_t are actually 64-bit non-negative integers.

Typedef unsigned _ _ int64 ib_uint64_t; II. Function flow

I probably took a look at the process as follows:

Trx_undo_prev_version_build / / Build a previous version of a clustered index record-> roll_ptr = row_get_rec_roll_ptr (rec, index, offsets); / / get rollback pointer-> rec_trx_id = row_get_rec_trx_id (rec, index, offsets) / / obtain TRX_ID-> trx_undo_get_undo_rec (roll_ptr, rec_trx_id, heap, is_redo_rseg,index- > table- > name, & undo_rec) / / get the previous version here, and put it in undo_rec-> trx_undo_get_undo_rec_low (roll_ptr, heap, is_redo_rseg); / / undo_rec as the outgoing parameter. Outgoing accessed undo-> trx_undo_decode_roll_ptr / / do roll_ptr parsing work to get segment id\ page no\ offset-> Open MTR to get latch ready to copy-> copy trx_undo_rec_copy-> submit MTR

The parsing tool is actually done by trx_undo_decode_roll_ptr.

III. Practical analysis

In fact, the analysis is quite simple, it is all written dead.

/ * Decodes a roll pointer. * / / from high to low is whether the first place is insert / / the 2nd to 8th bit is segmentid//, the 9th to 40th bit is page no / / 41st bit to the 56th bit is OFFSETUNIV_INLINEvoidtrx_undo_decode_roll_ptr (/ * = * / roll_ptr_t roll_ptr, / *!

< in: roll pointer */ ibool* is_insert, /*!< out: TRUE if insert undo log */ ulint* rseg_id, /*!< out: rollback segment id */ ulint* page_no, /*!< out: page number */ ulint* offset) /*!< out: offset of the undo entry within page */{#if DATA_ROLL_PTR_LEN != 7# error "DATA_ROLL_PTR_LEN != 7"#endif#if TRUE != 1# error "TRUE != 1"#endif ut_ad(roll_ptr < (1ULL >

= 16; / / move 16 bits to the right * page_no = (ulint) roll_ptr & 0xFFFFFFFFFFF / get 32-bit page no roll_ptr > > = 32-bit ibool / 32-bit right * rseg_id = (ulint) roll_ptr & 0x7FF / get 7-bit segment id roll_ptr > > = 7-bit right * is_insert = (ibool) FFF / * TRUE==1 * / / the last one} at this point, I believe you have a better understanding of "how to understand the Innodb DB_ROLL_PTR pointer in MySQL". You might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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