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Editorial Team
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Asked: 2026-05-12T21:28:19+00:00

MySQL table with couple of fields: id – PRIMARY KEY url – CHAR(255) .

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MySQL table with couple of fields:

  • id – PRIMARY KEY
  • url – CHAR(255).

url field is also unique and indexed. Currently have couple of hundreds of thousands entries in that table. MySQL gets really slow.

The idea is: if I add hash BIGINT UNIQUE INDEXED field, and create composite index hash/url – will it work faster? Means will MySQL first check my hash/url pair using hash and then url? Will it be faster?

If answer is yes – for what reason this is not transparently implemented for indexed strings in MySQL?

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1 Answer

  1. Editorial Team

    The idea is: if I add hash BIGINT UNIQUE INDEXED field, and create composite index hash/url – will it work faster?

    The key lookup on a BIGINT field is faster than on a CHAR field (surprisingly, performance gain is more noticeable on a key miss than on a key hit).

    Note, however, that a BIGINT hash has very high probability of hash collisions, that’s why I’d not recommend to use it in a UNIQUE field

    Means will MySQL first check my hash/url pair using hash and then url?

    If you create a composite key on (hash, url) and search for the hash only, it will use the ref condition on the hash part of the index.

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